It's not necessarily the fill valve; it could be something external.
However, it is conceivable that for one reason or another the percentage of water diverted to the refill tube has diminished. The cap does contain the nipple for the refill tube, so your suggestion is one possibility; perhaps the diameter of the orfice under the nipple has begun to narrow from accumulations of one thing or another. It's quite possible that replacing the cap will solve the problem. That it is happening on all three might say something about your water. Can't hurt to try.
First, however, you might just want to service the valve by cleaning it. That's free, although the caps are pretty cheap, too. Someone cited a piece not too long ago from Toto that gives pretty comprehensive instructions on servicing the Korky valve. They make it look a bit complicated; in fact it's easy. Basically, you are rinsing everything. Rather than repeat the instructions, here's the link. Valve Service Instructions Type B valve, obviously, is what you have. Give that a try and let us know how it goes. There's also a video on the Korky web site, but the Toto piece is more comprehensive. Korky 528 Maintenance Video