Defenders with low forms

[Toxcatl]

I have three 6-starred defenders who can play all 3 defense positions with aerial defense and defensive wall SA. All their highlighted skills are above 100%. Yet these three rarely rated above 6.5 in most matches. I win these games but conceded at least one goal per match even if I mostly play def counter. Any tips to improve them? Thanks.

Do they do their job or do you conceding a lot of goals? Are thay playing alone or do you have the 4th defender? Do you have DMC? What is their Fitness, Strength and Aggression?

Look, DC and DL/DR are very different positions, so it is a good idea to train them differently even if they can sometimes play as 3 DC.
DC and DMC are almost the same though, so it's good to have both these positions on the DMC.
If your defenders are outnumbered, they won't get good rating just because they're trying to do a very difficult task. You need to have DMC in your team, it is a very strong position. In TE2018 it is better to play 3-1dmc-* than 4-* IMHO, unless your opponent has a flanking formation and defending MC(s).
There are many other factors that may affect their performance, like hidden quality (if it exists lol).