As you say, there is no advantage to any of you from you buying out the house and moving to 2.5%.
If your partners agree, you can do a side agreement. Your relationship with UB does not change. The other two remain jointly and severally liable for the mortgage.
You take full responsibility for the mortgage and they give up their ownership rights. You pay the mortgage in full and they have no further involvement, unless you fail to make the mortgage repayments.
What's in it for them...
Not much, other than the payment you make to them now.
They remain liable for the mortgage and if you fall into arrears, their credit record is damaged.
They still have a loan, so their ability to borrow could be limited.
What's in it for you...
You get to own the house.
But you will have to pay them now for their share of the house.
This can work fairly well for family homes. Not sure what the tax implications would be for an investment property.
One solicitor told me that your partners can sell their interest in the home to you, while remaining on the mortgage. Another solicitor told me that this could not be done.
Brendan