I feel like 1/2+ε is enough for the 3 spider case. The spiders can start (or take their sweet time to get) in this configuration:
And then the two slow spiders can follow the red arrows to ensure the ant will be trapped, while the "fast" spider keeps watch on its edge.
Unless I overlooked something, this seems to work, but doesn't prove there isn'...
A couple of words are confusing:
By "hover" I assume you mean it's moving (generally means to stay in place)
By "retain" I assume you mean some kind of average speed needed. Clearly our speed will accelerate and not be maintained as a constant.