The Bible is full of patterns and the ones I see are always in three.
The tabernacle consists of 3 parts, Holy of Holies, Holy Place, Outer Court.

Each of the 3 parts has 3 parts.
Outer Court - has the doorway, altar of burnt offering, laver.
Holy Place - table of shewbread, lampstand, altar of incense.
Holy of Holies - ark of the covenant which has -
...3 inside it - jar of manna, Aaron's budded rod, ten commandments
...3 on top of it - gold plated mercy seat, two Cherubim, presence of God (cloud)
7 is perfection which is what the Holy of Holies represents the throne room of God.

The people could only go into the outer court.
The priests could only go into the Holy Place.
The High Priest could only go into the Holy of Holies.

Holy of Holies - why Holies? Rev.4:8 Holy, Holy, Holy, Lord God Almighty!

RJ