I've tried using those 3 along with an extra squash or jalapeno via the imitator. By the 3rd flag (of 4), I'm toast. I'm not even close. Most levels, I can get by with only 1 of the explosive units, even if there's the occasional bobsled or catapult thrown in. A whole level of them, though...
I did knock out Dr. Zomboss's Revenge last night (it actually wasn't too bad), so I'm down to 2 survivals and the !$#$#%@#! bobsledders. I'm at 5-6 in my second trip thru adventure mode, which I'm doing for Zen plants and stress relief.
Nice avatar!