Summoning three spiders is great in my opinion. If I can do it (requires having an 11+ tome and a soulstone) I probably will. The arachnids will come in with 2 wounds, but 3 arachnid with 2 wounds are generally better than 2 with 3 wounds you know. And Bleeding Edge Tech can heal them very easily.
A lot of the time you can only summon two though.