Are they related? Malocclusion can be genetic so dental issues with all 3 while unusual, certainly isn't impossible. I would definitely try to get a second opinion.
I am so sorry for the loss of one of your boys and I'm also sorry you're needing to syringe feed them, doing just one is difficult enough, 2 is definitely a challenging! Is it the supreme science recovery food you're using? I find that one needs diluting with a fair bit more water to not block up syringes, although that does mean you'll need to feed more to get the same amount of nutrition in. Personally I swear by a mix of burgess dual care pellets and the oxbow fine grind critical care if you are able to get hold of those, they can be mixed much thicker so makes for smaller, quicker feedings.
Don't forget to look after yourself while you're syringe feeding, being hungry and tired makes an already stressful activity worse and dont forget its ok to put them back for 5 minutes to calm down and take a breath for yourself if they're being difficult and resisting.