I maintained 3 long distance relationships simultaneously for over two years a few years ago:
A 20yr old Dutch girl from Maastricht
A 24yr old German Frankfurt
English girl from Birmingham (I'm in London)
You need to manage their expectations and be available on what whatsapp at all times, and see each one every two weeks for a whole weekend. Ryanair is shit to fly with but cheap, I would pick a return flight to Frankfurt for 30/50 or 60 pounds max, they picked me up from airports and I stayed at theirs, so cost wise was no big deal. I caught my usual flight back Monday at 6:30am and was at my desk in the city by 8:30am with a starbucks. Birmingham was actually more expensive go figure.
I don't do this now purely from a "been there done that", I'm getting older and lazier and can't deal with the hassle, one woman close by for convenience is all I can handle now.
Long distance needs planning and expectations management. Seeing each other should be pre-agreed and planned and stick to it, this is what causes most problems. When you're there, don't be shit, satisfy all her needs, when I saw my girls I was with them 24hrs for the whole weekend we partied, ate, showered and off course slept together.