I don't know ANTLR well enough, but my suspicion is that ANTLR has a lookahead predicates that allow to tell the lexer to only skip new-lines that are *not* followed by optional whitespaces followed by ///.
E.g. something like
WS1: ( ' ' | '\r' | '\t' ) -> skip;
WS2: ( '\n' { _input.LT(1).getType() != COMMENT}? ) -> skip;
You need of course a COMMENT production.
I've not tried it out, this is only a hint of a possible direction to solve this.
Cheers
Andi