She couldn't do MFL because she was filming MP. Sound of Music was filmed the following summer. There would have been no interference with her Fox films.
She had a deal with Disney that if Warner at the last minute decided to use her for Lady Walt would let her out of her contract.
But what was the third Fox film? I thought it was two.
It may have been a 3 picture deal, but it was nonexclusive and Andrews had three films released by other studios between TSOM and Star!: Torn Curtain [Universal], Hawaii [UA] and Thoroughly Modern Millie [Uni]. I suspect, but cannot confirm, that Fox let her out of the 3 picture deal after Star! bombed.But Julie had a three picture deal with FOX and wouldn't have been able to do MFL for WB.