From the Football Grounds Guide:
Crawley station is just over a mile away from the stadium. You can either take a taxi (about £6), or from the bus station across the road, you can take a Fastway number 10 bus to the ground (this runs every ten minutes). Otherwise it is about a twenty minute...
It's nowhere near Three Bridges station.
One of the Fastway bus services to Broadfield runs past the stadium, I'd have thought that, logically, one of them must stop at Three Bridges.
The nearest railway station is Crawley, but even that'll take a twenty five minute walk, I'd have thought.