It depends entirely on their ability and whether they have lived, or are living, in an English speaking environment now.
If the child's only English exposure is with you, then I would suggest continuing to drill vocabulary, and the grammar "I like" and that should be fine. Also, getting them to learn their ABCs by song and through puzzles.
If the child has English speaking parents, has lived in an English speaking country, or is living in one now, then more grammar is easily taught and maintained. "I like" "I hate (don't like)", emotions ("I'm happy, sad, mad, fine") and greetings (Good morning! Hello! Goodbye! How are you?) are good starters. Any grammar that includes an article (a/an/the) is a little too difficult for them, so if you want to teach more advanced grammar (I have a/an ___) (It's a/an ___) then you can't expect them to use the article yet. It's too intangible.
When you teach grammar, you should be using lots of hand motions, and if you know any of their language, saying it once in their language with the same motions is best. Just once though, so they understand what motion means what. Keep your motions consistent.
Typical motions I use are:
I = touch my nose
Mine/My = point my thumb at my chest
like = hug myself
have= grab something in the air and pull it back to my chest
don't = have your hand parallel and shake it in the "no thank you" motion
hate = mark an X with your fingers in the air
Three year olds are going to pick up some grammar, but the BIGGEST thing right now is getting their pronunciation right. Letting them hear their R's and L's, B's, D's and V's, and even their H's, P's and F's, especially if their mother tongue doesn't have those sounds. So I would suggest lots of ABC singing, ABC chant walks (my kids like walking a round path made of ABC flashcards or foam blocks).
Those are my suggestions.
