Sliver abilites and when they affect the Slivers
As soon as one of the Muscle Slivers leaves the battlefield, its ability disappears, leaving the other sliver smaller.

Static abilities on permanents create continuous effects, functioning as long as the permanent is on the battlefield, no longer and no shorter. They are not activated; they simply are. That's why they're written as statements of fact: "All Sliver creatures get +1/+1."

So for example, your opponent could block one of your 3/3 Muscle Slivers with a 3/3, and one with a 2/3. All the creatures deal damage to each other simultaneously. State-based actions are then checked, and the two 3/3s and the 2/3 have taken three damage and die. The first Muscle Sliver is now gone, so the remaining one is now a 2/2. State-based actions are checked again, and that sliver has two damage marked on it and now only has two toughness, so it dies as well.

Remember, this doesn't affect the amount of damage dealt, since all the damage was dealt at the same time, before anything died. So when you ask "does it stay at 3/3 for the attack phase?" the answer is... yes and no. It's still a 3/3 when it deals damage, yes. But it gets smaller immediately after.