i will be facing this very same dilemma in another 12 rows or so. i unfortunately don't know the answer for certain, but i have a thought on it and maybe someone can confirm or deny it for us.
the important thing with moss stitch is to knit the purls and purl the knits (as they face you) so my thought is, let's say you decided to K the 3 together; on the opposite side, that is going to appear as 1 purl stitch, so as long as you knit that stitch on that row, i think the moss pattern would be maintained. so i would p, k3tog, p, making sure you p, k, p the following row. in my head, this seems right, but i haven't knit it yet.
hope we figure it out! i'm going to be stuck too!