it's first time playing blood and i've got a hard time against cultist, i'm playing on the 3rd difficulty.

Is there something i'm missing ?

No, Blood is a hard game, but fair in my opinion. Cultists are very dangerous, and so are you. Try to save often and always look for cover, if there are too many of them. When you can take them one by one, shotgun fire can make them freeze for a brief moment, when you have the opportunity to fire again. Also one flare pistol shot, and hiding to wait for them to catch on fire is great, but slows down the pace of course. When there are a lot of them (preferably close to eachother) flare pistol alternative fire, explosives, napalm launcher, aerosol can, basically anything that fires at bigger area can take them out and you can enjoy their screams.

When you are close to them and you have nowhere to hide, you can try crouching. In my experience it makes harder for them to hit you.

If there is a blind corner, assume there is a cultist behind it. TNT is your friend. (Don't forget alt-fire to light it, and bounce it off walls)
If you see one way down the map, snipe it with flair gun
You can also snipe with Tommy gun with small bursts. It is a hit-scan weapon so the first shot should hit.
DON'T let them get into the water if you can at all avoid it.

Go paint the town red.

Thing is you will eventually learn from where they come from and react accordingly. They can't hitscan you if they are dead :)

Read the Blood Wiki to learn funstuff and secrets that will help you out through the Episode
http://blood-wiki.org/index.php/Main_Page