When President Reagan's choice for the No. 3 spot at the Energy Department arrives to take up his duties, he may find the post of undersecretary something less than he imagined.

That's because the expected White House choice, Pat Collins, 37, vice president of public affairs for the National Association of Home Builders, was not the candidate of Secretary Donald P. Hodel.

Hodel tried to get the White House to nominate his special assistant, Earle E. Gjelde, to the job, but Reagan administration officials felt that Gjelde's GOP credentials were not sufficiently imposing. Collins was administrative assistant to Rep. Ron Marlenee (R-Mont.) before moving to his present job in 1979.

While Collins may get the job, however, Gjelde may wind up with some of the undersecretary's perks and responsibilities. The Energy Department's regional offices, which have been reporting to Gjelde in the absence of an undersecretary, will continue to report to him, sources said. And Gjelde also is inheriting the undersecretary's office, with Collins slated to move into the smaller office Gjelde has been using, the sources added.

Collins will be left with responsibility for the department's conservation programs--which observers note has not exactly been a top priority of the Reagan administration.