Hello, a Pallas Gull was near Ueckermünde/NE-Germany for some days in 2016, allowing photographs with other european gull species, including GBBG:
(If worse, unsharp pictures of this Gull are needed, fell free to ask me)
I dont know if this is your question: it didnt stand out among the other Gulls by size to me.
If you need pictures of all three species together (which would be really exceptional, I agree with Nutcracker), maybe its good to ask BF members from Poland or other Baltic states or the relevant birding sites on the internet?