First, you should not have such a replica set configuration - arbiters are needed to make the number of voting nodes odd, yours is actually making number of voting nodes even! You should remove the arbiter and just run primary and two secondaries.
As far as what caused the spike in CPU, without logs from mongod from the time performance was impacted it’s impossible to tell. I would first look at the logs, then at any system logs that you might be tracking (were there issues with disk? other processes impacted?)
In particular, in mongod logs, I would look to see what operations may have started around the time issues first surfaced, or what operations ended when the problems stopped.
Asya