It's been several years since I've been to Dorst Creek, but based on what I remember...
It really comes down to which campsite you have booked. Several of the camp sites in this ground are actually fairly small - much smaller than you'd normally expect in a National Park campground. Other campsites are much larger, especially those on the outsides of the loops.
Depending on the exact site you have booked, 3 tents may be, at best, a stretch - especially if the sites next to you have more than one tent on them.
You can find a map of the campground here which will give you some idea of how large your campsite might be.
If you find that one of your tents doesn't fit, then I would suggest asking nicely at a few nearby sites (especially the larger ones) to see if they would be willing to share. Officially this is not allowed, but given that you are already staying in the campground you likely won't have too much trouble finding another site that would be willing to let you put up one tent. Also, keep an eye out on recreation.gov for available sites - people do cancel, and it's not uncommon to be able to snag something in the week or so beforehand.
Enjoy Sequoia (and if you get the chance, Kings Canyon) - they are a great pair of parks. If they weren't so far away I'd be there every few months :)