I've done both druid and cleric, but not shaman. Discussing druids healing in groups at higher levels:
Druids will keep up with clerics and shaman until 34 or 39, when clerics get superior healing + Complete Heal. We start falling behind but make up for it in versatility/utility as we have STR buffs, DS, regen, ports, evacs, etc. Clerics are also quite versatile, but on this server people view them as CH and rez bots, which is unfortunate. Druids are viewed as port bots, which is equally unfortunate.
Druids change a bit as healers once we get Regrowth and Chloroblast (our fast heal). We are good patch healers at this point. If you're with an enchanter, you get to heal them during charm breaks until the cleric's heals hit them. I've saved a couple of lives with Chloroblast
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Druids as main healers at higher levels DO NOT do well without mana regen. I've been grouping with friends from level 58-60, usually without a cleric. I'm commonly accompanied by a necro or two, sometimes an enchanter. Before 60, I required twitches from the necros after lots of pulls or many adds, on top of them mezzing, especially if the MT was a monk or something similar. If I had C2 I did not require twitches often, or only after refreshing regrowth of the grove, and I was able to charm the dogs in Chardok for extra DPS. If we wiped due to trains, we had to find a cleric to rez, or use an EE with necro rez. Outside of trains, I kept my group alive (save for the pullers, who kept going off into the distance and would get ice cometed to death - come on!) and generally >70% hp.
At 60, I have my own mana regen (+9 a tic) and don't need twitches at all. I have a big heal (Nature's Touch) as well as my fast heal (good for saving the enchanter after charm breaks), and I was able to keep a charm pet for extra DPS. I occasionally healed the shaman as well. With C2 and bard song, it's crazy. I can nuke, charm, and main heal at the same time. I also have succor, of course. Although the last time I cast that, the spell actually left me behind to die... :|
Clerics diverge from druids at higher levels. They cast less since their heals have more power, and in high level groups they cover healing and occasionally CC (druids do healing and DPS). They've also got a nice stun for casters, which I don't often see utilized for some reason...?? (I'm not sure why, I need to get my cleric to high levels to see what's going on.) And of course clerics get rez, which is great for post train recovery if you can get the bodies.
By the way, here are a few examples to show you what druids can do or not do with heals or in groups:
I've duoed Vessel Drozlin with an enchanter (healing the charm pet, I was in my high 50s)
Same enc, we required a cleric to trio the tangrin (the charm pet had 30k hp, I couldn't keep up)
Required a cleric to heal the monk tank when doing Ulump for the druid epic
Duoed corrupted brownie with a monk and did great
Duoed corrupted gorilla with a 45 monk (fear kiting!)
In CoM, I main healed, DPSed, helped pull with harmony
In Chardok I charmed a dog, main healed, succored in bad situations, and DPSed
Ignore the grouping druid hate on p99. It's nonsense. You can main heal as a druid for normal group/duo/trio sessions all the way to 60. Since you have a group of friends, you won't have to sit LFG for hours on end while people try to tell you to join DaP or go to bear pits. Learn efficient healing methods, learn to charm, learn to buff, learn to DPS, have a good mana pool, and you will be a good grouping druid.