Let's see if I have this right. You have the voltmeter connected across the two posts of the battery, then you engage the starter, and while the starter is engaged, you observe a reading on the meter of 3 to 4 VDC. Is that how you are doing it? If that is right, the voltage drop is excessive. But that by itself DOES NOT indicate a weak or defective battery. We need to know what the standing voltage of the battery is before you turn on anything or engage the starter to even begin giving suggestions.
Here are some questions to be answered.
How old is this battery?
What is the standing voltage of the battery (no load of any sort)?
Have you tried "jump starting" with a known good battery?
If so, does that provide good, energetic cranking, or does it still seem reluctant to crank?
Exactly what engine is in your tractor (get the model and type numbers from the engine data sticker)?
Certain types/brands of engine can exhibit starting/cranking behavior that mimic a problem in the starter, battery, or electrical system, while the actual problem is elsewhere.

You could get a similar reading if the engine is hydro-locked from a bad carb that leaks though and fills the sump with gasoline.

Remove the spark plugs and make sure the engine cranks freely.

A tractor battery gets to be suspect after a couple years. Some last much longer and some don't. Best to fully charge it and get it tested if in doubt.

The battery is about 6 months old with the standing voltage being 12.3V. I am charging it now as we speak.

I have not tried to jump start with a good battery. Could you please tell me in detail how to do that?

I will get the rest of the information that you guys mentioned and thanks again!

Jump starting a vehicle with a weak or dead battery.
The proper sequence for connecting to, and disconnecting from, each of the batteries, MUST be carried out exactly as stated, to prevent possible explosion of either battery, and personal injury resulting from said explosion. Equally important is to correctly identify the polarity of each post of both batteries ahead of time. Hooking the jumper cables with polarity reversed can damage one or both vehicles charging system and/or cause an explosion of a battery.
Position the donor vehicle, or good battery, near enough to the dead vehicle for your jumper cables to reach both batteries. The next step I like to do is strictly a safety measure to keep the loose alligator clips from touching each other while you connect to, or disconnect from, the good battery. Use a non-metallic piece of material such as a wooden broom handle (it can even have the broom still on it) to fasten the alligator clips of one end of the jumper cables onto. It can be anything non metallic if the alligator clips will fit over it and clamp securely to it, but I'll keep saying broom handle. By opening the jaws of the clips and attaching them to the broom handle, you assure that the alligator clips on that end will not short together. After attaching the 2 clips onto the broom handle, lay the broom on the ground or shop floor. Next, connect the free end clips to the good battery posts, red to positive or + post, black to negative or - post. Now, pick up the broom handle and take the red clip loose, attach this red clip to the positive post on the dead battery, this is the only clip you will attach to the battery, and remember that, CONNECT ONLY THE RED CLIP TO THE DEAD BATTERY + post. Next, unclip the black alligator from the broom handle and find a good, clean, bare metal surface on the dead vehicle engine that you can attach the black clip to. A place you can choose for this, if you can get to it, is where the negative battery cable attaches to the engine or the frame. Get as far from the battery as you can to make this last connection. It might cause a little spark when the clip touches the metal surface. It will not be necessary to run the engine of the good battery vehicle (if there even is a vehicle), just connecting the two batteries together this way is sufficient for the dead tractor to start. Next, try the key switch on the dead vehicle to see if it will start. Hopefully it will start. When you disconnect the jumper cables, take the black clip off the dead vehicle frame and attach it to the broom, then take the red clip off the dead battery and attach it to the broom, lay the broom on the ground. Now you can disconnect the other two clips from the good battery and store your jumper cables for the next emergency. Follow this procedure and you will not cause an explosion or damage either vehicle charging system. Even when practicing this very safe technique for boost starting, it is still possible that a defective (or frozen, if deep winter time) battery could still explode. For this reason, wear eye protection and sturdy gloves when jumping off a dead vehicle. If you are lucky enough, or "discerning enough" to own a set of "fool resistant" jumper cables that separate mid span, and have "correct polarity indicator LEDs" on each mating half connector plug, you are a very well prepared person.
If the engine still does not crank with a booster battery, then we need to check some other things (Bill Kapaun has already mentioned some).

I would just load test the battery, you can get 12V from a lattern battery. But due to the amount of cells or lead plates the LT battery has much more AMPs' of course unless it is dead but it still may read 12V but NO AMPS

V=IR, so if V=12V and I~0 then R is very very high. Why would R be such a high value. (If I=0 then V=0)

Just go buy a battery.