If i have 2 characters and one of them roll 1+ melee dmg and other roll 2 melee dmg can i claim them as 3 dmg???

Yes.

When you resolve dice, you can resolve all dice that share a symbol (in this case, melee damage). You resolve them one at a time, so if you had one dice with 1 damage and another with 2, you'd resolve them separately but end up doing three damage. This is important if you wanted to target different characters, for example, or in the case of something like Second Chance (you can use the first dice to remove Second Chance, and the second to do it's normal damage).

The exception is modified sides, which are resolved at the same time as one of your other dice, and are combined together. In this case, if you had a 2 damage side and a +1 modifier side, you'd resolve them together for three damage. Again this can be important for certain effects like Second Chance (in this case, the two damage with a modifier is straight up worse than two individual damage dice. There are cases where it's the other way around).

Thanks for helping me out

im glad that you bringed up second chance into this bcs i was wondering if u have second chance and i place ammo belt and my characther is dead and i remove 5 dmg and im left with five does ammo belt go away and i get to use second chance twice? Bcs i have second chance and two ammo belts isnt that too much if its possible?

Ammo Belt had been erratad to only work with Weapons and Equipment. Check out the Destiny product page on this site and there's a document with current rules changes and clarifications.

Rules Reference 1.6