Schweinebraten with lentils? That's a weird combination. At first I thought those were peas, which usually go with mashed potatoes
My family usually had Schweinebraten with Semmelknödel and red cabbage
The song is unrelated to the phrase as it is used in a Bierzelt. It goes "Oans, zwoa, drei, g'suffa" and sometimes, depending on the band, it goes on with "Prost ihr Säcke!" to which the audience replies with "Prost du Sack!"