To help you get started with improvising over the blues with a I, IV, V progression you will need to know which notes these are on harmonica in 2nd position.
The 2 hole draw is the I chord
The 4 hole blow is the IV chord
The 4 hole draw is the V chord
If you want a more detailed lesson about the 12 bar blues structure then you should go here.
Thank you for watching this little harmonica tip