Usually (though not always) you can show up with four adults for a table booked for three adults. This is because there is only some restaurants have tables for three; so when you book for three, you would be allocated a table for four.
You say an infant, and although someone may have the exact answer, I imagine that you would not run into any problems, as even if it was a table for three (and not four) there may be enough room to squeeze a little one in.
And even if it were a problem they may ask you to wait a little longer rather than seat you. You should try to call and add the infant as soon as you know.