Get an Aprilaire current relay #50 or #51. It clamps around the common
blower wire and senses the blower motor current. One switches 120
volts, the other switches 24 bolts, I don't remember which is which.
Each relay will carry 1 amp.

That being said, the best solution would be to run a load calculation
to determine how much air each room needs and resize your ducts to get
the right amount of air in every room. Note that is not a job for an
amateur. It would be the best solution to your problem.

Most "booster fans" are propeller fans. They do not produce the high
static pressures needed for duct systems and are not very effective.
You may not want to run it that way for the computer room in the winter
since it is too hot there anyway. In the summer run the computer room
fan 24/7. Just plug it in to a wall outlet.

The bedroom fan unit can be on a relay. The relay can be mounted
inside the furnace control cabinet for safety.

Good luck.

Stretch