Three weeks ago, as CBS prepared to broadcast a television movie dramatizing Mitch Snyder's unrelenting struggle to get shelter for the homeless, the activist helped organize a glitzy fund-raising dinner at the swank Dominique restaurant.

But yesterday the glitter was gone and it was back to bare essentials -- three loaves of homemade bread and a plastic container of water, to be exact -- as Snyder and 26 followers gathered in the dingy and sweltering homeless shelter at Second and D streets NW for a last meal before starting a fast.

It is the third Snyder-led hunger strike in about a year and a half, and this one is aimed at forcing the Reagan administration to release immediately the $5 million it promised 2 1/2 months ago -- after a 33-day fast -- to renovate the decaying shelter operated by Snyder's group, the Community for Creative Non-Violence (CCNV).

When the agreement between Snyder and the White House was announced March 16, the administration agreed to transfer ownership of the 1,000-bed shelter to the District government and to provide $5 million for renovations that Snyder says are essential to keeping it open and habitable.

But the funding is bogged down on Capitol Hill, as the houses of Congress have weighed in with competing bills on the mechanics of the transfer.

Yesterday, Snyder said the architect and contractor hired to renovate the shelter have told him that unless work begins almost immediately the building will not be completed in time for the winter.

In the last fast, Snyder took only water, lost 47 pounds and was said to have had a dangerously low potassium level. Snyder said this time he will not even drink water, which will speed the effect of the fast. He said that is necessary because the administration must act quickly.

He said that after 10 days, four of the other people fasting also will stop drinking water. "It's not just a matter of waiting me out and the administration saying, 'Bye-bye with Mitch,' " Snyder said.

A White House spokesman said yesterday that the administration has lived up to its commitment to Snyder and hopes that the House and Senate can resolve their differences so that the money can be released.

Snyder said the administration's attempt to shift the blame to Congress is unfair. "Let me make it clear that there's no one else responsible but the administration," Snyder said. "They've got the $5 million and could cut the check tomorrow."

Sen. Mark Hatfield (R-Ore.) has included a provision that would transfer the building title to the city and provide the $5 million in an emergency supplemental appropriation. Under a House bill, Snyder said, jurisdiction would be transferred to the District but the site would revert to the U.S. government if it stops serving as a shelter.