Cellular access only

Plan comparison:  https://simplisafe.com/features-alarm-monitoring#monitoring-plans

I'm not sure about the text messaging alerts with the Standard Plan (14.99) - you'd have to call SS to ask - however, included in the plan (according to the comparison page), fire and burglar alarms would be monitored and dispatch available with that plan.

The cons of the Standard Plan, at least that I can think of:

Cannot use the phone app
Will not have plan features available from your online account (as opposed to the Interactive monitoring plan/24.99)
Cannot use 'secret alerts' setting for sensors
'Medical' alarm/dispatch not available with the Standard Plan
There could be others I'm not familiar with...

Yes, you can operate your SS3 system on cellular-only (and no additional fees for cellular), but you won't be able to get OTA (over the air) firmware updates without wifi.  You could, however, take your base station and keypad(s) to a friend or family member's house with wifi, to update firmware. (keep in mind, if you get more than one keypad, ALL keypads must be updated, individually).

And it's important you ask SS about the text message alerts with the 14.99 plan (Standard plan), since you'd want them if you don't have the phone app (the phone app requires the Interactive Plan) - sorry, I just cannot remember if text messages are included in the Standard Plan or not.

Should you decide to purchase SS3, please make sure to call first and ask if the base will have the new 4G cellular module pre-installed, you'll need it due to the upcoming sunset of 2G for major cell carriers.  SS has Verizon and ATT cellular module/SIMs, when on the phone, tell the rep whichever one gives the best coverage in your area.