scubadobadoo:As others have said, diving with three STINKS. If you must, then make sure your kids understand the dive plan, emergency plans, and stick to them. Also, making sure that the threesome makes eye contact FREQUENTLY with ALL members of the group helps. Make it clear that each of you is equally responsible for the other two divers and that this will take a little extra work underwater than it usually does with two divers. One more tip, diving in a single file line with three divers makes it hard to see everyone. I suggest trying to dive like the blue angels, side by side by side, when possible.
The position of divers in relation to each other is always important. Diving single file, reagarless of the number of divers works better when lights are used to communicate, though that can be difficult in brightly lit waters or when divers don't have a suitable light. Though there are exceptions, ascents and descents with three are probably best done in a star formation (all three divers facing eachother) but that also works well with two divers.
As long as you pick a leader, have all three divers at the same depth and basically side by side (exceptions for light communications prevbiously noted), there is no good reason for diving in a three man team to "STINK".
And don't let them get in front of you together with you behind them watching them because it makes you feel better. That will only lessen their safety by promoting a "dad is in control" feeling that doesn't really exist underwater. Basically you all need to feel equally responsible and in control.
It's a bit hard for a dad to feel like there is an equal sharing of responsibility when diving with a 13 year old. No? Is it even possible for a young child to have a level of responsibility equal to that of their parent? I think the father is the supervisor by default in this case and that fact as well as his ability to provide that supervision and guidence in the water is best considered right from the start.