Tiphook PLC will become the world's third-largest container leasing company with its acquisition of Sea Containers Ltd.'s fleet of more than 200,000 dry cargo and tank containers.

If the integration of the new assets is completed as expected in April, Tiphook willdouble its size overnight to rank behind only Itel Corp. and Genstar Corp., both located in San Francisco.Sea Containers' shareholders approved the $537 million sale last Thursday.

"We have gained, at a good price, the assets of Sea Containers that we wanted. This, combined with the current high level of utilization which both fleets enjoy, means that enhanced earnings will come through very quickly," said Tiphook's chairman, Robert Montague.

The doubling of Tiphook's container fleet will be achieved with only a small increase in administrative costs, resulting in strong cash flow, greater interest cover and a continuing low tax charge.

Transportation market analysts believe Tiphook made a wise decision.

Peter Bergius, of Kleinwort Benson of London, said the acquisition presented a "very encouraging" outlook for Tiphook.

He noted that Tiphook is doubling its fleet without adding any capacity to the world market and at substantial cost savings through integration and rationalization of its increased assets.

Mark McVicar, an analyst with County Natwest Securities Ltd., viewed the acquisition with equal enthusiasm.

He said the price was "sensible in current market conditions."

Both analysts predicted that Tiphook's pretax profits, which are expected to rise from 10 million ($16 million) in the 12-month period that ended last April 30 to 31 million this year, will soar to 75 million to 80 million in 1991 after the takeover.

However, Mr. McVicar also noted that Tiphook's investment carries some risk because the shipping industry on which it depends can be an erratic business due to the vagaries of world trade.

Although Tiphook will not acquire Sea Containers' 80,000 specialist and refrigerated containers, which it had sought when it initially entered the bidding contest, these are widely viewed as of secondary importance to the assets it will take over.

Mr. Montague said the acquisition would be a major step in the further growth of the group, which was founded only 12 years ago.