On a 1-to-5 scale, grade inflation is tempting. But giving 5s everywhere teaches you nothing together. A real 3 is high praise: 'I could marry this.'
A 3 is not average
In online reviews, three stars means disappointing. Here, a 3 means you're happy: this is good, it works, it's a solid base for a life together. It isn't settling for less than you deserve.
Saving 4 and 5 for the exceptional
A 4 is really special, something you'd find in maybe 1 relationship in 20. A 5 is once in a lifetime. Keep them for that and they still mean something; hand them out freely and they mean nothing.
Using the same scale
When you both use the scale the same way, honesty feels safe. A 3 from your partner reads as a warm yes, not a lukewarm grade.