I need to do passport control and again security control for second flight?
Is 3 hours enough?
No one knows. No connection is every guaranteed so best we can do is give you a likelihood. If all goes well 3 hours should be fine. Things that are most likely to go wrong:
- Significant delay or schedule change for your first flight
- Wait times at immigration. This depends a lot on your time of arrival and what else is happening. Recently very long wait times have been reported (example). It's unclear whether these are outliers or regular occurrences at the moment.
- Complication at immigration: if your paperwork (passport, visa, etc) are complicated or not fully in order, you may get a lot of question or they will need to double check some info.
- Security to re-enter. Again, recently there have been reports of very long wait times (example). Again, it's unclear whether this happens a lot. To mitigate this one, you can buy FastTrack access. I don't know whether this is actually faster, or just another marketing ploy.
Stansted recommends to be checked-in and head two security at least 2 hours before departure.
I'd say you have an about 90% chance of making it. However, if you miss it, you will likely lose your second ticket completely and may have to buy a new ticket on the spot and you are stuck in Stansted until the next available flight. Personally I don't do self connections under 4 hours, but that depends on your own risk tolerance.