[unknown] Never put a ship in the Museum just for income. They are not worth it. Your goal is to keep using your ships as long as possible. This is the only way to cover their cost.
You cannot put a 3 stars in the Museum. They have to be upgraded to 5 stars first. Once in the Museum, you cannot use them anymore so, think twice before putting a ship in the Museum.
We can keep an infinite number of ships in dry dock. I usually will keep a ship there 1 or 2 months before sending it to Museum. I keep a maximum of 1 slot in reserve for large exploration and send only 2-slots once in a while.