Online gameplay hardly uses any bandwidth at all; you could probably get by just fine with only 1 gig per month if you're only looking at playing online for 10-20 hours per week.
That said...
The biggest issue with data for gaming isn't bandwidth, it's latency, jitter, and packet loss. While the actual bandwidth usage for online play is not very high at all, it requires a very stable connection with low latency and low jitter, which are all things that cellular hot spots tend to be pretty bad at.
Latency is the time it takes for your computer (in this case the PS3) to start receiving the data it has requested. For gaming, you want this to be very low. Ideally you want this to be under 50 milliseconds though most games remain at least moderately playable up to about 150 milliseconds or so.
Jitter is the variation in latency. For example, just because you may get a latency of 25 milliseconds for one request, that's not helpful if the next request takes 400 milliseconds to answer.
Packet loss refers to whether pieces of information get lost between the host and your machine. "Lost" packets have to be retransmitted, which can reduce the quality of the gaming experience by causing some lag.
All of these are areas where mobile hotspots tend to have their issues. Of course, many report good experiences with mobile hotspots these days, so I'd check to see what providers other gamers are using for their mobile hot spots to find out if one provider is best.
