My baby just is gaining .3 Kilos a month, she's breastfed. Is that .3 Kilos a month acceptable?

As Evelyn mentioned, sounds like a good weight to gain per month! Personally I am not too bothered by my baby's weight as long as he is peeing and pooping enough and is drinking his milk as he should. The percentile is just a generally guideline. I feel that it's best to just focus on the healthy of your baby and ensuring that he's happy and healthy