Replacing one out of three leisure batteries is not ideal.
The remaining two older batteries will have different electrical characteristics to the new one. The outcome may be that the bank of batteries does not charge to its full potential.
There is also a possibility the other two batteries have been damaged by the failure of the third.
I would suggest there are two options.
If the remaining two batteries have sufficient capacity to meet your needs then just use those until they need replacing. Take care to insulate and make safe the connections to the failed
battery.
If you require the capacity that three batteries provide I recommend that you buy three new batteries.
Don't forget that Alpha Batteries provide a discount to members.