A good attack - only option is Lightning Reed. It attacks multiple plants at once. It's pretty weak on its ownpretty weak on its own but 2 of them should take care of the trio without a problem - WHILE attacking other neighboring zombies.
Also, while this isn't a straight up "attack" answer (for that see @Ben Brocka's good one, or #1 in mine), but a good combination to defeat the camel zombie is with Iceberg Lettuce + some other attack plant.
This works (and is special enough to mention as a separate answer) because when the front zombie in the camel triplet steps on an iceberg lettuce, all 3 zombies in a trio freeze!
This slows them down enough that lighter attack plants (2 pea shooters or a repeater) can handle them easily.