If all your domains point to the same single site then you do not need the £3 multi domain option, included in the basic hosting fee you can point as many domains as you like to the one main website for no extra cost. So domain3.com and domain2.co.uk could be pointed to the site on domain1.net for no extra.
You can move them before or after, it really depends how best it would work for yourself.
For the included free shared SSL your secure address would be something like:
https://vulcan.unitedhosting.co.uk/yourdomain.com/
Let us know if you have any other questions,