Introducktions take time and they are not predictable.
My first thought would be to keep the muscovy separate till you feel confident (perhaps by having a fecal test done) that there is no risk of illness for your pekins.
Aside from that, two drakes and one female duck could put her at risk if the muscovy drake takes an interest in her. I hope others with both pekins and muscovies check in here on this.
So I add members to my flock by keeping them separated with a fence at first, then when it feels right, supervised time together, but never alone together until I am absolutely certain they are getting along. Sometimes they backslide and I need to reseparate them.
Bless you for taking in the lone muscovy. I hope it all turns out well. Please keep us posted.