Obvious thing but just to be sure it is mentioned, get the best pet focus item you can.
Pet focus was kinda squished out/nerfed a while back, but every little bit helps. The best group drop item currently is in Crypt of Sul called "
Shade's Phase". It off a mean ass camp that needs CC...just FYI.
As for group make up... entirely depends on what you want. I am one of the weirdos with an army of alts to mix and match the group make up, and have access to friends accounts to extend that pool of possibilities even farther. I generally like to run with 2 potential tanks... whether they be pet or plate. Had it too often where a tank can be dead/out of play and wipe the rest of the group with out a back up (mez/stun/charm/surge DPS faster then heals, what ever).... and I look at healers the same way (either 2 healing classes or one with a back up cleric merc) But then I am in no hurry to get things dead ASAP, but like to live through the fight =)
As Rusty said there, you can make a BL tank work... but it involves some time investment in learning the class well, and tweaking your stuff.
I have used both RD and KISS as my healer macros and both do well, after spending a bit of time making sure things are tweaked to where I want them. Since each group make up is a little different, each healer's INI may need fine tuning to fit the group you have them playing in, along with their capabilities. (Tinkering now with getting more cleric stuff put into RD, though I really ought to get this PC authorized so I can download an RG compile for it.)
I currently have a friends BL I am using with RD and just getting over 104. His gear is still blah and barely anything over autogrant just yet, but his pet has handled adds and pulls aggro well enough when I turn taunt on (gearing him up while leveling in TBM) and is using a focus from Shard's Landing still. It can be a slow process with a lot of bumps and deaths along the way, but I find it fun to see what works and what doesn't, along with seeing what other folks have figured out as well =)
Working out a 3 toon combo can be rough... there is always that fight that seems exactly designed to not work well with your combo, heh. (Hence my obsessions with having a lot of options =P ) Sorry I tend to talk a lot of times....Anyways... how I figure out group make up is figuring out strength and weaknesses and then matching what I plan to do with the toons best suited. For instance, if you plan to use a cleric merc + healer, I would recommend a druid or shaman (the shaman being the better healer IMHO, with speed group heal + group heal + AE healing wave + group HoT) Or.. if you are finding that is not enough , a live cleric will out heal a dru/sha and for longer.
Both dru/sha classes get a form of push-root (paralytic spores/spray) and so can help with CC if your enc/bard is having issues (provided the target can be rooted of course) with shaman getting the second line of single push-root (Virulent Paralysis) and when the cleric merc is staying on top of the healing, the toons can assist with DPS.
As for bard or enchanter... both do well as enhancing the groups capabilities in thier own ways, and CC. Again, kinda depends on what you are needing. Bard are limited in how many they can lock down continuously compared to an enchanter.... but they are excellent puller and can reduce the need for mezzing in the first place. An enchanter is more... straight forward in setting up. Sorta lock N load then forget, heh. You can do the same with bards, but it kinda depends on whose macro you are using and how well you set it up... bards to to shine best when played live I find.
Course.. just my opinions tossed in the mix =)