I installed a hot tub and when I connect the white wire from the gfci breaker to the neutral/ground bar it trips, otherwise the tub runs fine

Do you have a white/neutral from spa to the neutral terminal on breaker, as well as the coiled pigtail to the neutral bar?

Yes

At the spa, can you unplug the loads separately? Blower, heater, lights, It may be doing its job and you have a ground fault.

I will try that, thank you!

I will try but the tub was working fine at its previous location.

Both the black and white wires from the hot tub need to connect to separate terminals on the breaker. The only connection to the panel neutral bar is the coiled white whire from the breaker.

Neutral and ground cannot be connected to the same buss.

If you need a 120/240V for the hot tub, then you should have a four wire feed to the receptacle.

Neutral and Ground can only be tied together at the main service panel or a main disconnect switch.

Donf is correct in that ground and neutral need to be on their own bars. Still it would not cause it to trip, remove load from breaker and verify it does not trip, it shouldn't, if it does, it is a bad GFI breaker. Did you disconnect loads at the spa to determine if it is the Blower, the pump, the heater, really need to know or do.

It calls for a 3 wire 240v, so what is the proper connection?

GFI Neutral(coiled white), connects to the neutral bar, Nothing will connect to the neutral terminal on the breaker.
Does it trip with the load wires disconnected?
In my first response, you mentioned you had a neutral from the SPA?
2 hots and a ground is 3 wire, 2 hots, a ground, and neutral is 4 wire.