It's a general rule, has nothing to do with duty periods or the like.
You fly 3 types of helicopters and, if you're dual rated, one plank and one helicopter, that's it.
Some countries outside EASA only even allow two helicopter types to be flown.
Read on AMC2 ORO.FC.240, it exlains a bit.
Back in the days a similar JAR OPS rule existed:
JAR-OPS 3.1030 Operation on more than one type or variant
(a) An operator shall ensure that each crew
member does not operate on more than three
helicopter types except that, with the approval of
the Authority, the crew member may operate on
four helicopter types, provided that safety
equipment and emergency procedures for at least
two of the types are similar.
(b) For the purposes of sub-paragraph (a)
above, variants of a helicopter type are
considered to be different types if they are not
similar in all the following aspects:
(1) Emergency exit operation;
(2) Location and type of safety equipment; and
(3) Emergency procedures.