3 seat sofa bed is a construction of two parts. Firstly there's a futon mattress and it sits on a futon frame, the second part.
The frame usually uses hardwood timber such as Oak or Beech. That said there are Pine frames in use but generally these days that's not the case.
Perhaps the most ubiquitous are Steel frames with mesh decks or sit/sleep sections. These "futons" are very inexpensive to make and are a perfect example of a product for the mass market.
On the other hand there are some very fine steel futon frames that are generally sold in specialist shops. These frames have higher specifications and use European steel i.e. thicker and stronger. The decks have "flexi slats" that distribute weight evenly. Models such as these are more expensive but still represent good value for money as they are virtually indestructible.
Lots of consideration should be given to the futon mattress. Years ago it was relatively easy to find comfortable futon mattresses. Whereas these days there are only two large scale mattress makers in the UK.
Another fact is these 3 seat futon sofa beds can have arms.
Most noteworthy, a 3 seat futon sofa bed can use a thicker futon mattress. This is because the mattress folds along its length once.
The mattress can use layers of various types of filling or combinations of filling. Examples are Wool, Cotton, Coir and Latex.
The mattress cover is mostly made from 100% cotton drill or a polyester/cotton mix.
Buttons or ties then keep the filling in place.
In many countries it's usual for the futon mattress to have a removable cover. The cover may have a zip around 3 sides for easy removal. The UK didn't adhere to the fashion for covers.