The Federal Communications Commission said yesterday that cable television systems can escape rate regulation if a community is served by three or more competing over-the-air signals.

The deregulation will affect 75 percent of all systems serving 90 percent of the cable audience in the United States, according to the National Cable Television Association. But the new FCC rule will not go into effect for nearly two years under the provisions of the Cable Communications Policy Act of 1984.

Under the act, guidelines were established for regulating cable television systems in such areas as ownership, channel usage, franchising, rates, service and equal opportunity employment requirements. The act essentially ends basic rate regulation on Dec. 29, 1986, except where the FCC determines there is "no effective competition."

The FCC determined that effective competition exists if, in addition to cable television, viewers in a community receive three television signals or channels with conventional antennas.

In a separate action, the FCC rejected complaints by Action for Children's Television (ACT) and the National Association for Better Broadcasting directed against several television stations for allegedly airing program-length commercials designed for the child audience.

The complaints concerned a number of cartoon programs that ACT contended were nothing more than long commercials advertising certain children's products. ACT said the programs violated the FCC's existing rules and policies.

The commission said it found no violations of FCC rules and policies. It said the broadcasters had generally followed FCC guidelines that require a clear separation between programming and commercial advertising material.

The FCC also said profit-sharing arrangements -- under which, in return for airing a program, a television station shares in the profits from the sale of products bearing the name of the program -- were an innovative way of program financing and distribution.

ACT had complained that the profit-sharing techniques would unduly influence the licensee's judgment, leading to selection of children's programs only on the basis of profit incentives.