I think you've answered your own question. The clue is in the 2 other computers.
If this is wireless, the proper answer is probably to do with reception on the problem computer. Especially if it's in a different room from them, perhaps further away.
You should try to optimise your reception and avoid interference.
1) raise the router above furniture level
2) Experiment with channels (some will work better or worse depending on your environment)
3) If you can detect strong neighbouring wifi, use a channel 5 stops away from strongest.
4) Relocate cordless phone base or video sender etc.
5) Be prepared to move the computer (or at least turn it so your body is not between the router signal and the wireless adapter's antenna.