Boars can be right little toads and the 6 months Terrible Teens is a prime time for them to start with the dominance issues .
They're all quite young though .
If Thatcher died today
it;s too early for Toby to do anything except grieve . Let him have his peace just now with the other hogs in chattering distance .
Is there any chance of re-bonding Colin and Trevor ? You'd need to go right back to basics , completely neutral, remove ALL Their smells .
Bath them , good strong shampoo ( I used Gorgeous Guineas Cedarwood and Lemongrass for this ) then in an empty pen with a huge lump of hay and veg.
If you go down the boars&sows route , you're looking at 3x castration, (about £60/pig ) recovery and no guarentee though if you go to a Pig Savvy vet it's a quick operation. (Then you don't have the issues with impaction )
But sows aren't all sweetness and light (my GP8 steadfastly refused to bond with our other pigs and attacked poor GP6 our placid boar)
It depends what you want long term. Do you want three cages of Mr&Mrs ? That would be lovely and not too much more work than three single boars ......though boars are tidier .
There are worse things in life than a little boar with side-by-side company . Our boar pairs didn't cuddle up, they liked their own space . But they liked to know the other one was there .
I felt sorry for GP6 when he had to side-by-side with our sow pair (GP7/GP8) but for his own safety he couldn't live with them.
He missed the bonded life (he'd had GP4/GP5 to live with)
He did get to spend the last 3 months of his life with GP7 (I'm not sure what she thought of him, he was a cheeky little git but she told him 'nope' ) they were like an old Darby&Joan couple until he died in January and she died in April.
You couldn't have Toby + a sow if you do pair up your other boars , they would fight more with a female in the midst .
I think in an ideal world , I'd give Toby a few days , see f you can re-bond Colin and Trevor then put out some feelers for a boar for Toby , a tiny piglet would be easier .
When you think about all the what if it doesn't work I think you'll drive yourself daft ( Been There Done That ) 
Then you have the what do I do when another one dies working out the partnerships
Who is in the photo ? Is that Thatcher ? Lovely boy .