Prudential Insurance Company of America is starting three speculative suburban commercial developments here in which it plans to invest $40 million.

Construction is expected to begin within two weeks on the largest, a 15-story, $20-million office building on Rte. 7 at Tysons Corner, called Tower 7.

A quick start is also planned on the smallest, a 5 1/2-acre warehouse industrial complex at the beltway and Shirley Highway. Prudential will spend $3 million there on three buildings.

The third project is a 50-acre industrial park off old Lee Highway at Prospect Avenue, on which $15 million to $20 million in development is planned over the next five years.

All three projects will be developed by Prudential's Capital Real Estate Investment Office, said Robert Nolan, a company spokesman.

What makes the projects unusual is Prudential's decision to act as the developer, and to build the three on speculation rather than for specific tenants.

Although ground-breaking for the office tower is two weeks, no leases have been signed, Nolan said. Prudential says it will take 18 months to build the tower, and only 18 months more to lease its 290,000 square feet of offices.

Prudential has never developed a suburban office building on its own here, although it was a partner in a joint venture three years ago in the same neighborhood.

Nolan said the giant insurance company decided on the Tower 7 project, "because we're in the mortgage business and making loans out there and we had funds available. The third factor is that we can't buy completed properties."

There aren't any commercial projects on the market, he explained, so Prudential is building its own.

Prudential is in a better position than most builders to tackle speculative construction, because it can finance the projects through its own funds or with pension money it manages.

Nolan said Capital Real Estate will begin the Prospect Avenue project with four buildings totalling 100,000 square feet. By 1983 about almost 600,000 square feet of building is planned for the site which was until recently a riding stable.