Since it's a small number, it makes sense that it's not going to increase with each level. This is compounded by the fact that OF crew member levels are also generally less impactful than their AF counterparts. However, it's also a crew member with no penalty, so that plays into their growth rate as well.
But eventually, that 3 will become a 4 at which point they become 33% more powerful.
Protip: If you do care about your OF goods for whatever reason, try to always include at least one of the +health crew members so you rarely or never have to repair your sub.