Engineer Specialist 3 cant seem to get any repair

I had the same issue, but managed to finally complete it, it's a bit hit and miss with the challenges currently. Only tip I can give you is try to complete the challenge by doing all the required repairs in one match.

Or wait until a fix is there as it's a known issue as you can see hereChallenges not tracking | EA Forums - 12743240

But as the message is more than 2 months old it seems not to be at the top of their priority list to fix