is this sentence in the past perfect "She'd sustained several injuries."
Yes (had sustained)
is this sentence a third conditional. "If she had, those boots definately wouldn't have stayed put."
what is the use of the third conditional here
A third conditional is a condition in the past, that obviously can't be met. So it's a hypothetical.
If I had known you were coming I would have baked a cake.
If I had been in town I would have had lunch with you.