Manchester Airport - How early to arrive?

Not sure if you need three hours but Manchester is one of the worst airports I've gone through. Security can also be a nightmare.

Additionally, there is a lot of construction at the terminals. They are being refurbished.

While I usually arrive at airports around two hours prior to flight time, I give Manchester an extra 15-20 minutes just because it's Manchester.

I used this airport just last week. Security was fine, didn't take long. I would suggest getting there around 2.5 hours before your flight time. There are trains at that time from Manchester Piccadilly and as well as the 103 bus above, there is the 43 bus from Piccadilly Gardens into the airport bus station. Both are supposed to take around an hour at that time of the day.
Train would be first choice though. Check the times on www.nationalrail.co.uk

5am or slightly earlier should be OK, but I'd advise against leaving any time later than that. Manchester airport station is a short walk from Terminal 2, so you should get to the easyJet bag drop-off area approximately 25 minutes after your train's departure time.

It varies, but the time needed for easyJet bag drop-off could be more than 30 minutes—and getting through the notoriously unpleasant Manchester security can easily take 45 minutes. Then it could take you more than 15 minutes to walk from Security to your gate.

It could be worth considering purchasing T2 Security Fast Track, for £5.49:
https://www.manchesterairport.co.uk/at-the-airport/security-fasttrack/

This ostensibly gets your through Security more quickly—and usually it does—but if the area's not as busy as anticipated, or if the Security team on duty is not managing the queues efficiently, it can take longer.

The first 103 bus of the day is actually at 0250, then 0350 and 0450.
The 43 bus runs 24/7/365, every 30 minutes overnight.

To access Platform 13 use the Platform 10 entrance in the main train hall, then down two moving walkways. The first of those walkways (a flat one) has been static all this week (and this morning) but the second one (which is inclined) is working.
From the Platform 13/14 waiting room there is an elevator (lift) to platform level.

Thanks everyone! It doesn’t sound like anyone has any qualms about walking to the train or bus at that hour. Time and date shows sunrise to be approximately 4;45 so it shouldn’t be totally dark. And thanks for the helpful directions about getting to platform 13. I really appreciate you taking the time to reply.

The UK is very safe and I have no qualms about walking anywhere at any time. If you can handle San Francisco, Manchester will seem very easy :)

It wasn't clear from your initial post that it was the safety aspect that concerned you. Manchester, like the rest of the UK is safe. You might see a few people that have spent the night outside, either by choice of necessity, just don't engage with them, you'll be fine.

I'd probably aim to get to the airport around 6am. For an 8am EasyJet flight to Belfast, that should be plenty of time, even with a checked bag.

Manchester can be busy in the early morning, but 3 hours feels a bit overcautious for a domestic flight. I'd rather have 2 hours than 3.

As for Piccadilly, I'd be fine walking there at 5am if it's only 10 minutes and you're comfortable with your luggage. Otherwise, a quick taxi is always an easy backup.

I'd rather have an extra hour in bed than spend it sitting in the airport.