My brother had a developmental delay with speech. At 3 he would say mum, dad, bub and that was it. Next thing mum knew he came out with a full sentence and she was gobsmacked. He was fine since, A student etc and is fine now.

Ears are another thing to get checked - as in glue ear and fluid levels.
One of my cousins' daughters wasnt talking much at all and I think she was about 3 when they realised she needed grommets in and had been having touble hearing, but could manage enough for no one to notice that they could be the cause up until then.

She'd been following basic instrucitons and could get her message across but not "proper" talking for her age level.