We need to realize this is all in flux, as SpaceX is changing everything, sometimes daily.
BN3 and the other bits they built for Boosters were attached to manufacturing bases, with different attachment points.
BN3 is currently on a test stand that was designed to hold a Starship upper stage. They built an adapter for the Booster connection points.
For BN4, the one they installed all the engines on, there is a manufacturing stand that has 20 hold downs.
It has yet (as of this writing) to be rolled out to the pad. The new launch pad (as of this writing) is also not yet complete, so it is not certain exactly how that will work out.
There are a large number (20) of hold down clamps in the base to keep the booster connected. This is the current approach, if there is any constant here, it is change. So expect this to change.
(After I wrote this, they had rolled out the launch table and lifted it to the launch stand. And then they rolled out the BN4 booster to the pad. No doubt by the time I finish writing this, they will have lifted the booster. Or flown it, who knows at this pace.)