Ross Perot spent an estimated $4.9 million to buy ads on three networks last week, close to $4 million at NBC alone. The Reform Party presidential candidate bought 300 15-second spots on NBC , in prime time, late night, daytime and weekend sports. The move comes after the FCC rejected Mr. Perot's bid for access to half-hour slots for ads on network TV (see story on Page 63). The candidate also bought 17 :15s at Fox, for approximately $400,000. At ABC, Mr. Perot spent more than $500,000 last week on :15s. CBS information wasn't available at press time.