Bankroll questions aside, three counters sitting beside each other is a really bad idea. You will all be varying your bet sizes in tandem - that's just going to look too obvious
DONT DO IT!!
Earlier comments on this thread are correct though - if you are all sitting at the same table then you are overbetting. like it was said earlier, it would be just as if you were spreading to 3 hands at the max +ve count i.e. instead of betting 50 you would be betting 3x50 - this is going to create less variance than if you are betting 1x150 - but nevertheless it would cause a significant increase in your ROR (risk of ruin).
Do you need a bigger bankroll if 3 people are playing simultaneously but at different tables? Yes in some way it is like one player playing three sessions -however since you will all be playing at the same time it is possible that all three of you hit happen to hit a losing streak - if so you may all need to adjust your bet ramp (just as you would ahve to do if you have 3 significant losing streaks in succession). If the 3 of you would be playing in succession, then there would certainly be no need for an increase in BR. But I would think that since you are playing simultaneously you will need to increase your BR. I dont know how to calculate as to how much you should increase it.
To be honest if your BR is only $5k then you should just play for fun. Best case scenario you are going to make $7.50 each per hour

. But if you are going to use this as pratice for when you have a $25k bankroll, then that's a good idea (that's what i did!).
If you are restricted to playing one table - why not have two of you flat-betting and counting, and one of you getting really drunk and drunkenly betting $30 behind each of the two guys playing when the count is good?