well... as long as they arent spindermites you should be okay. You said 2-3mm long and that is longer than what i know any spidermite to be. I take it you are growing outside, and yes, spiders will be beneficial. Just keep an eye on your plants over the next few days. If you really wanna kill some pests, get a praying mantis. Protection, and a badass show as it bites the head off of any poor bug that gets too close!