We have 3 days in Brussels. Is Flanders Field a good day trip from there? We have also considered Bruges.
All of Belgium is pretty much daytrippable from any other point in the country. And because all the major rail lines converge on Brussels, it's even easier. For the remnants of the battle fields around Ieper, I would make one of two recommendations. Either rent a car or take a bus tour. Although the town of Ieper is nice and compact, the battlefield memorials are spread piecemeal across a very large area, and interspersed with small hamlets and farms (it's not like a Civil War battlefield, where they preserve everything in one). You need some kind of wheeled transportation to move between sites. There's enough written on Brugge elsewhere that I probably don't need to elaborate.
I haven't been to the battlefields in Belgium. But you might want to investigate how they might compare with other war sites, like perhaps the Normandy beaches, or the cemeteries in France and Germany. If the poppies aren't blooming the month you're there, they may look like farmland. Not to overstate the comparison, it's striking how many war memorials there are in Belgium in comparison with the U.S. No only is there a town or city monument, there's a "profession/trade" monument in the main Post Office, the Central railroad station, at the main Police Station, in each neighborhood. Unless there is a particular family connection, you may wish to balance travel time against the scale of the experience available. AFAIR, in the Brussels Cathedral (a nice visit ... ) there is a memorial plaque to the Scottish (??) regiment which liberated that part of the city at the very end of WWII. I'm not "recommending" it, because it's at least 1/4 as powerful as a concentration camp visit, but there's a Nazi prison camp just outside Mechelen, Breendonk, which is very, very close to Brussels. It is tricky to get to by public transportation. It's run by the government, and has audioguides in English. It's more about terrorizing the locals into collaboration than about extermination, but many died there.
"If the poppies aren't blooming the month you're there, they may look like farmland." That's because most of the landscape of the Ieper salient IS active farmland. As I mentioned above, although there's small cemeteries and memorials everywhere, only small remanants of the war are preserved here and there. But these are well worth visiting.