Those plastic 3-way bypass valves are pretty commonplace and not normally an issue. I would say it was either water trapped inside or potentially (although rare) compression clamps that were over-tightened at the factory which cracked the fittings. Perhaps it got cold enough for the anti-freeze to freeze? The winter was extra nasty this time. I assume the anti-freeze was undiluted.
You don't need to use 3-way valves. You could do it with 3 regular valves instead. If you need a little more PEX tubing after cutting out the old valves, regular household PEX is fine. I'd get a PEX tubing cutter - they don't cost much.
There are 6 different ways to connect PEX tubing.
6 ways to connect PEX I use copper crimp rings in our TT but only because I already have the crimper tool. For only a few connections, Sharkbite is the way to go plus they can easily be pulled apart if ever needed with the special tool.
You can also remove the compression clamps by using a pair of pliers (with flat nose) and wiggling the crimped piece that sticks out back and forth until it loosens up enough. You can use a small screwdriver if needed to pry it apart. Or just cut the tubing...
Sharkbite would definitely be the easiest but they're the most expensive and I'm not sure if available in a 3-way valve. Seatech makes a marine plastic version which is available at Dyersonline:
Seatech 3-way valve Not bad at just $13 (before shipping) or $13.50 on ebay with free shipping. Likely available at local marine stores for 10 times the price, lol.