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).
Well, the MC rules definitely says the extra attacks granted don't add together.
You can read the first sentence as describing the paragraph, the second sentence as describing what "don't add together" means, then the third one clarifying that it works the same way with similar features.
Or you can read the first sentence as saying "you pick one extra attack feature to use and ignore the rest", the second sentence as being redundant and meaningless, and the third sentence extending it to similar features.
Or you can read it as the rule making clear that the wording "instead of one attack, you can make X" doesn't work if you are already making more than 1 attack. I mean, naively, you could with 1 extra attack feature apply the feature, go from 1 to 2 attacks. Then on the first attack, you apply the feature again, turning that 1 attack into 2 again; so, just as you cannot apply
extra attack when you already have more than one attack, you can't apply the fighter's version and the ranger's version, because the perquisite (one attack) is no longer holding.
Assuming you are a level 17 character: fighter 11/BS 6.
Interpretation 1: You can make 2 weapon attacks and cast a cantrip.
Interpretation 2: You can make either 3 weapon attacks, or 1 weapon attack and cast a cantrip.
Interpretation 3: Still unclear. Does the "moreover" only apply if you had used the first part of the feature, or does it apply independently?
Similar dancing of wording applies to the haste attack action. Does it mean "you cannot make more than one attack as part of the attack action" or does it mean "you can do nothing else in the attack action except make one attack" or "you can do whatever you want in the attack action, so long as the result is exactly one attack". Those are all reasonable readings of the haste attack action restriction, and they result in subtle differences in how it interacts with the bladesinger ability (on top of if how you connect moreover to the rest of the bladesinger extra attack feature).
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Myself, a Fighter 11/BS 6 is free to make 2 attacks and a cantrip as part of the attack action. It is T3 and you built your character for just this trick. Have fun. I don't see a serious balance hit here, nor is the wording so tortorous I'd recoil.
I'd also say that Haste is "the only thing you can do is one single attack in the attack action", which bars making a cantrip attack in the hasted action. Haste is already pretty darn strong, it doesn't need the boost.