We will be travelling this coming summer with our son, who is 22. Normally, we would get two rooms, but considering how expensive Amsterdam can be, I am instead looking for places that have rooms for 3 people, on the lower to medium end of the cost spectrum (which I know will not be that low in Amsterdam). I know Haarlem will be less expensive and am willing to stay there but would still like to give it a try to find something in Amsterdam. Also looking for Delft recommendations.
Not sure of the pricing, but in Amsterdam you might try—
Hotel Alexander - near Vondelpark. We stayed here with a Rick Steves Tour in 2013 and on our own on another trip. Not sure how far in advance they are booking. You may need to email them directly for summer info.
Hotel Wiechmann- I think mentioned in Rick,s book. Have stayed there several times.
Yays— has several locations. Rates were very reasonable when we stayed there in month of February, but not sure about summer rates. We stayed in the one they refer to as Salthouse Canal on Zoutkeetsgracht. They do offer discounts off standard rates. They are a few blocks from a tram.
Also, you might consider the town of Leiden which is a short train ride away from Amsterdam. We really enjoyed staying there.
You probably will get better responses if you post your question in a more appropriate section -- Destination Q & A, Netherlands. You are currently are posted in the "Review" section. And when making reservation make sure you request a triple room.
In Spring 2019, my young adult daughter and I stayed at the XO Van Gogh in the Museum district, across the street from the Van Gogh Museum. They have triple rooms.
The hotel was well-priced for the terrific central location. One part of the building is a hotel and another section is a hostel. So look carefully on the website when booking. Our twin room with private bath in the hotel part was small but cheery and clean. There were a lot of young people at breakfast, but otherwise we would not have known there were hostel rooms in the building.