Question about end caps on Pex lines for Washer in new house
If those were crimped-on end-caps you'd have to cut them off - but they look like "sharkbite" fittings, which can be removed. If you lack the special tool, close an adjustable wrench down to the size of the pipe, and use it to push the tan collar into the fitting.

Search for "sharkbite adjustable wrench trick" or "how to remove a sharkbite" if that's unclear. If all else fails, cut them off. Try to be sure the water is shut off first.

99.9% of washer hookups are "garden hose thread" so I suspect 3/4 pipe thread would be the wrong choice.