more than just (m:n)?

Hello,

I need a relationship between say table A and table B having primary
keys pkA and pkB respectively.
It seems like a (m:n) (if I did get the idea of this kind of
relationship right). So a table AtoB may be like;

CREATE TABLE "AtoB"(
"fk_pkA" integer NOT NULL REFERENCES "tblA" ("pkA"),
"fk_pkB" integer NOT NULL REFERENCES "tblB" ("pkB"),
PRIMARY KEY ("fk_pkA","fk_pkB")
);

What I want to accomplish is to have either 0, 1 or 3 (not 2 or more
than 3) of a particular fk_pkB value.
Now if I add another column and a CHECK;

CREATE TABLE "AtoB"(
"fk_pkA" integer NOT NULL REFERENCES "tblA" ("pkA"),
"fk_pkB" integer NOT NULL REFERENCES "tblB" ("pkB"),
"chkcol" text CHECK("chkcol" IN('a','b','c')),
PRIMARY KEY ("fk_pkA","fk_pkB")
UNIQUE("fk_pkB","chkcol")
);

This makes sure that no more than 3 rows with the same pkB value but
still allows for 2 rows.
Any ideas?