The note a chord is named from is called the root note (like the root of a tree). A good way to memorize a chord progression is to play only the root notes of the chords all the way through the progression. An example of this is shown below.
Since chords 1 4 and 5 in the key of G are G, C and D, you will first need to locate these notes on your harmonica. To begin with, G (1) can be found on holes 2 and 3, C ( 4) can be found on holes 1 and 4, and D (5) can be found on holes 1 and 4. These notes can also be found in other places higher up on the harmonica, but stick to these ones for now and get to know them thoroughly.
Once you are confident you can instantly find the root notes of chords 1 4 and 5 , try playing through a 12 bar Blues using mainly root notes but using occasional other notes as well. This is demonstrated in the following examples. As your playing develops, you will be able to move freely between many different notes and use the root notes as landmarks to come back to when you want to emphasise them.