It should not be a problem to book such flights, even with stopovers. Usually airlines list cost in miles for one ways and do allow bookings with stopovers, but they charge more miles for such routes. Another issue - availability for miles.

The problem might be the lack of miles; you do not have enough miles on one airline or partner airline that would cover 3 tickets to Europe and back .

If I were you, I would not use miles for the US domestic flights, unless these are from coast to coast or a connection in US (without stopover) on the way to Europe. You can get US domestic flights VERY cheaply.

See if you can transfer miles (with small or no cost) to one account/one airline program. Airlines... try Scandinavian (SK)/UA/ LH - they're all star alliance members

good luck
R.