I'd say none of the proposals.

"neither" can sometimes be used for more than two options, but I wouldn't use it here, but if I was forced to:

Neither proposal A, nor proposal B, nor proposal C is acceptable.

Better: None of the three proposals is acceptable.

Thanks a lot to Dragon and to opine. Yes, now that I see it in writing, it is quite clear to me.

Cheers

According to a traditional usage rule, neither is used only to mean "not one or the other of two." To refer to "none of several," none is preferred. Example: None of the three opposition candidates would make a better president than the incumbent.