If someone defaults in a contract does that terminate a contract?

A contract can specificaly provide for what happens on a default.  Assuming it does not, and one party breaches a material term of the contract (the contract may define what is a material breach - otherwise it is a little nebulous, but generally it means a breach of an important term going to the purpose of the contract) the non-defaulting party generally has a choice.  He or she can either rescind the contract, in which case neither party would have any further obligations under the contract (not all contracts can be rescinded, depending on the extent ofperformance has occurred under them) or continue to honor the contract and seek damages (generally with a lawsuit) for the other party's breach.  Thus, if the other side has breached a material term of the contract, you may not have to perform under it anymore.  However, it is very doubtful that the other side will just agree with you that they have breached the contract, or, if so, that it was a material breach, and the issue may have to be decided by a court.  Also, you write that the other side has defaulted on the contract three times.  If the same provision was breached each time, it is possible that, by failing to take any action the first two times the contract was breached, you have waived your right to enforce the provision which you claim was breached.