In my experinece it comes down to whether or not the "cutters" can keep the others busy. At this point in my business I'm training my help to prune, they don't climb yet. Now, and only recently could I have a 3rd man on a fine pruning job. If it is just me on a fine prune can't keep one groundie busy, and I mean busy busy.
As far as the financial side. You need to do an hourly cost analisis. Basically take your annual expenses, projected if you don't have them, and divide them by your billable man hours. It will vary, but what it comes down to is the difference in cost between a 2 and 3 man crew is payroll + maybe a little fuel if he is running a saw... The more men you add, the more your "breakeven" cost comes down. If you are able to maintain your hourly rate with 3-4-5 men, that's awesome, and it just means your profit margin increases.
For example, if I remember correctly I can break even around $55/man/hr for a two man crew if we're working 20 hrs/week. (I based calcualtions on half time because we are growing, it's more like 30+ on average now). Goal is $75/man. If we add a third guy, I still aim and push for $75/hr, however I know deeep down inside we still make similar profit at $65/man (I'm just shooting numbers, don't do the math and correct me....).
I would love to be getting $100/man, I think we'll get there, I'm still training... So much about it comes down to skilled help!