Hotels are complete "units." They can be sold as such, or a player can receive houses as "change," providing there are enough houses for this purpose.
In this case, the most "change" you can receive is two houses. You have to sell one hotel outright, and the equivalent of four houses in each of the remaining two properties, putting one house on each of those.
I'm assuming that this will allow you to pay off the debt, and you'll have a little bit left over. You can't buy any more houses until at least one is made available. You may unmortgage other property with what money you have left.
Another solution is to offer most or all of your mortgaged property in exchange for either cancellation of the debt (by the landlord), or to a third party for enough to pay the debt, or in "chunks" to third and fourth parties. (If it's a two way game, you're lost anyway). In a multiway game, people will have to worry about third parties, and that may give you some leverage to make a deal that will help them, more than it helps their opponents. On the other hand, if your hotels are the "big threat" in the game, others may be happy to see it disappear.
For future questions, it might be helpful to know which monopoly your hotels are on, what your other properties are, the number of other players, and their holdings.