NOTHING about this makes sence....
Your car CANNOT EVER, blow the heater core withough first blowing the radiator cap or the radiater UNLESS you have a faulty heater core.
By design alone a heater core is much stronger than ANY radiator siple because it's made up of the same components but in a smaller more compact field that naturally lends more strength... Besides all that...
I've never had a dealing with the three tube heater core you have to deal with. The easiest way to fix it is do a simple flow test out in the yard or dirveway.
Hook up your garden hose to each of the 3 tubes in order, and view the results.
One of three things will happen:
Water comes out both the other tubes,
in that case you can cap the tube you want, and use the other for the return to the block.
Water comes out only one of the tubes,
this means that the heater core serves two entirely separate duties and it contains a valve the prevents fluid movement in one or more directions... Which is highly unlikely.
The third thing that might happen is that water could come out both of the other tubes regardless of which tube the water is pushed through... In that case, You can cap ANY of them and use the other two for heat.
There is no reason for a heater core to lose heat other than loss of fluid movement or loss of fluid temperature period... That pretty much sum's it up. If you have no water movement then you will have no heat... If you have no hot water, then you have no heat as well...
I hope you get it settled.