Toronto press baron Lord Thomson of Fleet has become the third bidder of the Toronto-based FP Publications newspaper chain.

Thomson shocked Canadian publishing circles with a $139 million bid for the FP chain, which owns the Globe & Mail and seven other Canadian dailies. The Globe is Canada's most influential morning daily.

The bid through Thomson Newspapers Ltd. is about $20 millin higher than a bid made last week by a group including Conrad Black, who controls Massey-Ferguson Ltd., a Canadian based worldwide tractor producer.

The Black group offered about $102 million for FP more than a month ago. Globe Chairman R. Howard Webster, a Montreal financier, jumped in with his own bid of about $109 million, leading to the Black counteroffer.

Lord Thomson, son of Kenneth Thomson, who founded the Thomson chain, recently settled bitter strikes which struck down his British publications, the Times of London and the Sunday Times.

The Thomson newspaper chain owns 120 daily and weekly newspapers in the United States and Canada.

Black and associates own Sterling Newspapers Ltd., a chain of small community publications.

The Thomson family has profited greatly from investments in North Sea oil. The Thomson chain had a profit of $29.3 million in the first half of 1979, up 15 percent from a year earlier. The British-based International Thomson Organization reported a profit of 44.1 million pounds, more than $110 million in Canadian funds.

FP is expected to report a loss for 1979 following the closing of the Montreal Star late last year.

The Montreal paper folded after a 10-month strike and left the rival morning Montreal Gazette as the only English-language daily in that city.

Although the Thomson chain includes more than 100 publications in North America, acquisition of FP would raise its daily circulation by about 50 percent to 2.4 millin from 1.6 million.

Shareholders of privately owned FP will meet next week in Toronto to consider the Thomson and Black offers, both of which expire Jan. 11. The Webster offer expired last week.

Control of FP is split among Webster, the McConnell family of Montreal, the Siston family of Winnipeg and the Bell family of Calgary.

FP was formed by Bell and Siston interests. Webster sold the Globe into FP for shares in the company.

The McConnells sold the Montreal Star to FP, also in return for stock in the chain.