Very Disheartening Week

[vomdogg]

Hey everyone -
First time posting.
I've been beekeeping for just on 3 years. This is my 3rd spring.
After last year finding out the hard way about swarming hives and not being properly prepared, my main goal this year was to be 3 steps ahead at all times - got into the hive early in spring, looked super healthy and approx 75 - 80% full, so have them a completely empty box on-top of a queen divider. They got in there straight away, checked them 3 times over 6 weeks , didn't see any queen cells, still plenty of room in the brood box ( added 2 empty frames )
Then this week, out of what I thought was nowhere - 3 swarms !
I caught all 3 luckily.
I just can't see where I'm going wrong.
My main hive is still looking pretty strong, maybe I didn't give them enough room ? I hadn't really thought of doing a split before, but should that have been my option with a strong larger hive ?
I have done a beginniners bee course, I've read so much, watched so many YouTube videos, spoken to other people with bees...
Is my hive just really healthy ?
Please, send me some advice, I'll take criticism also.
Thanks.

[charentejohn]

Sure others will have proper info, especially re splits etc. as I am just starting so no direct experience of those.

One thing I was told was when I asked about the size of boxes was that they will swarm when ready regardless of free space.
Hopefully someone may have some insight into that.   Good news is you caught them so that is a good thing.

[paus]

Did you see an empty Queen cell?  My guess, your swarms did not come from your hive

[TheHoneyPump]

I was thinking same thing. 
Consider the possibility that those three swarms are from elsewhere.  FREE-BEES!
You say your hive is still strong.  A hive that has cast 3 swarms (multicast), would be left with a substantially smaller population.

To help figure that out look into these things:
- Is/was your queen paint marked?
- are there eggs in the hive now?  Other brood stages?
- are open queen cells present?  Look for cells that have the end precision cut and flipped open, like when opening a tin can.