i bought some fruit trees recently in the 3 gallon pots. I want to upsize to 7 gallon but some of these fruit trees got fruits already, is it a good idea to change container and will it affect the fruits?
No that fruit wont be much anyway. Repot and prune accordingly.
CL23 - It depends on what kind of 'fruit' we're talking about. It's ok to pot up and expect minimal effect on deciduous fruiting plants, but you should try to avoid repotting when deciduous material is in leaf.
Al, what do you mean by "when deciduous material is in leaf."?
and by the way, the fruit trees are a couple of citrus and a guava
Deciduous drop their leaves in the fall. By in leaf, would mean, visible leaves and actively growing.
Below I've shared a great link for a thread you may want to look at. It has tons of great info!
On the second page, around March 19th, you will find some info on Citrus and Guava , for the questions I had about the tree's. I have both needing re potted. :-)
I hope it helps.
Here is a link that might be useful: Tree's in Containers