I have [[[Experiment One]]] on the board with one counter on, making him a 2,2. I cast [[[flinthoof boar]]] using a mountain and a forest, making the boar a 3,3. Does this trigger [[[experiment one]]] , giving him another counter ? Or is the boost to the boar applied only after the boar resolves ? ( it's a static ability, right ? )
Yes, Flinthoof Boar's continuous effects will apply before the game checks for triggered abilities.
It's when replacement effects are thrown into the mix that things get a little tricky: Replacement effects apply before other continuous effects, but they look ahead to see what some of the others are planning to do. Once replacement effects decide how this affects their application they are applied the permanent enters the battlefield, causing continuous effects to apply, then finally notifying triggered abilities that it has entered the battlefield.
614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c-d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent's characteristics on the stack (see rule 400.7a), and continuous effects from the permanent's own static abilities, but ignoring continuous effects from any other source that would affect it. Example: Voice of All says "As Voice of All enters the battlefield, choose a color" and "Voice of All has protection from the chosen color." An effect creates a token that's a copy of Voice of All. As that token is put onto the battlefield, its controller chooses a color for it. Example: Yixlid Jailer says "Cards in graveyards lose all abilities." Scarwood Treefolk says "Scarwood Treefolk enters the battlefield tapped." A Scarwood Treefolk that's put onto the battlefield from a graveyard enters the battlefield tapped. Example: Orb of Dreams is an artifact that says "Permanents enter the battlefield tapped." It won't affect itself, so Orb of Dreams enters the battlefield untapped.
611.3c Continuous effects that modify characteristics of permanents do so simultaneously with the permanent entering the battlefield. They don't wait until the permanent is on the battlefield and then change it. Because such effects apply as the permanent enters the battlefield, they are applied before determining whether the permanent will cause an ability to trigger when it enters the battlefield. Example: A permanent with the static ability "All white creatures get +1/+1" is on the battlefield. A creature spell that would normally create a 1/1 white creature instead creates a 2/2 white creature. The creature doesn't enter the battlefield as 1/1 and then change to 2/2.
603.6a Enters-the-battlefield abilities trigger when a permanent enters the battlefield. These are written, "When [this object] enters the battlefield, . . . " or "Whenever a [type] enters the battlefield, . . ." Each time an event puts one or more permanents onto the battlefield, all permanents on the battlefield (including the newcomers) are checked for any enters-the-battlefield triggers that match the event.