How clearly can you understand your three year old? I can understand my son, but there are some words where he replaces certain sounds with easier sounds. EExample - he will say "ock" instead of "off". "Res" instead of "yes". He can say the letter "Y" when he says "yellow", but "yes" is hard. It is a hit or miss on what sounds he can say. He is currently in speach therapy, but I am curious. He will be three on Apr 28th. He is on point with language, but behind in articulation.
Re: Articulation in your three year old
my son also has an artic delay and sounds exactly like your child. he struggles with sounds in first position although he usually gets them in middle/last position.
there's a huge variance when sounds develop in kids. this chart may be helpful for you. it's not uncommon for y/f sounds to be poorly developed for a 2.5 year old.
https://www.bisd.us/sharp/English_Developmental_Sound_Chart%5B1%5D.pdf
This made me laugh because we had the opposite problem. DS replaced C with P. We felt bad for his friend Cooper. He grew out of it though. I think the OP's son is pretty normal. It seems like all kids have one or two sounds they have difficulty with. DS is almost three. Every once in a while, I still have to translate toddler-to-English, but it's become much less frequent.