Along with the advice about, since you have about 6000 US and about 6000 UA miles, figure out which one (and ONLY one) of those two programs you want to earn in from now on. Then make sure you have that FF# in your reservation for the UA and LH flights. You won't get miles for the Condor flight as far as I know, but between the IAD-MUC, FCO-FRA and FRA-IAD flights you can get about 8000 miles or so (I didn't look up the exact figure).

US and UA do not "operate together" but they are partners, along with Lufthansa, in the Star Alliance. Nobody should spread their miles across different programs within the same alliance (unless they've already hit top elite level in one of them or under some other weird circumstances). They also do codeshare some flights (e.g. your "United" flights home which are actually operated by Lufthansa) but the flights don't have to be codeshared in order to qualify for frequent flyer credit.

So pick US OR UA, and from now on ONLY use that one number at reservation or checkin time whenever you fly US, United, Lufthansa, Air Canada, or any of their other partners.

I'd pick whichever of the two (United or US) that you think you'll be flying most often in the USA. There are some benefits to flying your "home program's" airline, so if you mostly fly UA, start crediting all your flights to UA. If you mostly fly US, credit everything in *A (including UA flights) to US.

If you're not sure or it's about equal, I'd suggest UA due to their overall better IMHO program. And due to free Economy Plus once you reach 25,000 Elite Qualifying Miles (EQMs). UA gives it free to all its own elites but no longer gives it to US and other *A elites. Since you're a big guy, flying UA as a UA elite would give you more room than US in coach. US allegedly has free unlimited F upgrades domestically (unlike UA where you have to "pay" with upgrade coupons good for 4*500 miles of travel earned every 10,000 miles), but US upgrades are getting hard to get. If you mostly travel UA and manage to get UA elite status, even the lowest earned level Premier (2P), at least you know you'll almost always be in an extra-legroom E+ coach seat.