IT WASN'T really a third gymnasium that the austere Senate was about to create for itself, Louisiana's Bennett Johnston protested before the vote the other day, because basically the Senate only has one full gymnasium with swimming pool and so forth now -- poor things. The Senate's second so-called gymnasium, Mr. Johnston explained, is merely an exercise room (with equipment, of course, plus separate locker and shower rooms). So why not go ahead and appropriate the mere $736,400 needed to finish up its third gymnasium, which is only, as he tells it, the Senate's second-and-a-half or maybe its second-and-a-quarter gym?

Make sense to you? No? Well, try this: and besides, since the thing has now been begun, not to finish it as a gym would be wasteful and anyhow some day someone would finish it and the cost by then would be higher. What's that you say? You still aren't persuaded? What are you, anyway -- some kind of Communist? The Senate, in its infinite, selfless wisdom, did buy the argument by a vote of 50 to 48 on Tuesday. So there will be yet another gym for the 100 legislators. No staff, no family, no friends. Just 100 senators availing themselves of these luxurious facilities. What a joy -- makes you feel physically fit all over just to think of it.

Or, do we mean physically ill? Sen. Proxmire, with whose words we don't always agree, had it absolutely right. As you might imagine, he led the opposition to this boondoggle, and in the course of doing so, he said this: "For the past year and a half, this Congress, and especially this Senate has been trying to hold down spending. Think of the sacrifices we have imposed. We have cut federal assistance for disadvantaged schoolchildren. We have hammered down appropriations for the handicapped -- men, women and children. We have cut food stamps. We have reduced Medicaid. We have forced elderly people to make higher payments for their Medicare. . . . almost all Americans have been asked one way or another to make a sacrifice. And yet, now we will provide over $700,000 for a third Senate gymnasium and over $1 million for a new especially fancy and super-equipped media hearing room (yet another legislative self-indulgence scheduled for this glitzy new Senate office building ). Why do we need still another gym?"

There was no good answer, but that will not surprise you. And it probably will not surprise you either that many of those who voted for this latest gift to themselves are among the world-class "economizers" on the Hill -- when it comes to economizing on other people's medical benefits or food stamps. Nor we suppose will you exactly faint from shock to learn that there are others on the list (which we print for your edification) from the party that has made such a big thing of Mrs. Reagan's life style -- of which we hope to hear nothing further from these gym-builders.

You hear it said a lot around this town -- we sometimes say it ourselves -- that it is really too bad that the people who work so hard in government, whether in Congress or the executive branch, are so dismally misunderstood by the public, so suspect, so vulnerable to bum raps. Then, just when you're full of sympathy, they go and do something stupid and insensitive like this. Three quarters of a million dollars to finish up a third gym for the same 100 senators who already have two gyms. How come the new gym won't also have an espresso machine and a wine bar? Or did we miss something?