Hi, I am new to the game and i have few questions regarding Base rules of 8th edition.
1. The rules say that i can shoot to all units as soon as I see at least a part of one model. But what is part? Lets say i see just the tip of a model sword who is behind a tree. I can fire to the entire squad?
That is correct.
2. A cryptek model say: friendly units within 3" of this units... this mean that i need to have ALL the unit models inside 3" to receive the power effect, or one only model? Or it mean that only the models inside 3£ get the power? Some powers speak clearly of models, some other only say units. so, what is the difference?
Most powers, or Auras, say either within or wholly within x number of inches. This can be models or units.
If it is a unit, any part of that unit has to be within range for the whole unit to gain the effect. This can sometimes lead to squads conga lining back to pick up a buff. Also not, if the range is 3", the edge of a model being 2.9 inches away will still give the entire unit the buff.
If it says "wholly within" then all of the model/unit must be within the entire range to gain the buff
3. Necron monolith is armed with 4 flux cannons... every flux ark is heavy 3. So i can fire 3x4 = 12 times every turn and i can also use the particle whip? All in the same shooting phase?
That is correct. Also, each gun may fire at a different target or the same Facing, and firing arcs are not a thing in 8th ed. So, if you have your flux cannon facing away from the enemy squad, you can still shoot them with it.