Arrive at STN 3 hours before a flight after the early morning rush ? Better have a very good book to read...
Better safe than sorry especially when using a point-to-point airline! Anyway, the coffee options here are better than they used to be (and there's a pay-to-use lounge too) that they used to be once you've passed the shops.
Bag drop might be still quite busy ... and it all depends on how many security channels are being used (a variable that is not necessarily passenger-dependent). In any case, if the OP is arriving by public transport, s/he is not always in control of their arrival time at STN ... as is this case using coaches.
If you have plenty of time, don't want to pay for a lounge, have no trouble walking for 5-10 mins, and want a bit of peace and quiet, make your way to the Ryanair pier (gates 41 to 59 I think) and head towards the end of the pier where there's usually plenty of space.
Good idea in principle .... but I've now twice been sent back because my flight departed from the former domestic pier (gates 81-88) where there is very little space and limited daylight!