Grubs are (mostly, anyway) inactive over the winter. I noticed a very few when digging in the gardens that were awake, large, and in charge. They didn't live long. ;-)

You would have been more likely to see the issue at the end of last summer or the beginning of fall if it were grubs, but it's still not impossible, of course. If you can dig and verify that it is grubs...well, do that and get back to us.

Late winter and (now) early spring have been really wet and irregular this year. The temperatures were swinging wildly through March, too, to the point that two of my butterfly bushes frost-heaved right out of the ground, breaking the base roots as they went. Those base roots are as thick as my thumb... I've never seen anything like that before.

It's possible, though also not likely, that heaving and cracking damaged the grass in a few areas. I noticed one or two areas, quite small, where that happened, but I have a newly renovated lawn and it's a bit thin on the southwestern side.

You can reseed, although spring isn't the best time to do it. If it's a bluegrass lawn, it'll fill itself in, eventually. 3 square feet is about 1.7 feet on a side, a bit much for KBG to do in a single season, although feeding it will help. Fescue and rye won't repair that fast or that well and will require reseeding.

You could pull plugs from the inevitable areas where the grass grew into your gardens or onto the foundation. Keep them watered for about a month and they should take.