餐馆 = 饭馆 饭店 is smaller

In fact ,they are all the same. 餐 and 饭 mean meals，the difference is 饭 also means a principal food, rice. 店and 馆means a selling place，the use of 店 is more extensive.

they are almost the same, but very little differences. maybe 餐馆，饭馆 is smaller than 饭店，I mean the space. Maybe 餐馆，饭馆 just cost one floor, but 饭店 maybe the hole building. 餐馆 is more formal than 饭馆。

Big restaurants can only be called 饭店, such as 北京国际大饭店. Smaller ones can be called 餐馆，饭馆，饭店, such as 洋洋餐馆，茂荪馆饭馆 ，双塔老饭店. And there's a 餐厅. 餐厅 means a restaurant or a dining room in a building. The building can be your house or other buildings with other functions. 俄罗斯飞象西餐厅

餐馆，饭馆 are similar, they are usually small.饭店 is usually big. But of course you could find lots of small restaurant called themselves 饭店。