One of the key aims in Year 3 is for children to write grammatically correct sentences. This starts with capital letters and full stops.
There are two ways to support your child with writing good sentences. The first is to read to your child. When you read you naturally pause at full stops (or question marks, exclamation marks, etc). It really benefits the children to hear this cadence of English (which is why we too have a class book which we are reading to the children). The second way would be to support your child when they write their weekly spelling sentences. This is also a good time to focus on handwriting.