Levistus's_Leviathan
5e Freelancer
You are correct. RAW, you must choose between three attacks or one attack and one cantrip.It is also not totally clear if you as a fighter 11/bladesinger 6 can make 3 attacks and replace one by a cantrip or if they need to chose which extra attack feature they use.
Thhe multiclass rule states, that they don't add together. I think you need to chose between 3 attacks or 1 + 1 cantrip (and one attack as a bonus action if you are an eldritch knight).