We're only about a month into the 1990-91 television season, but that's long enough to see a few trends developing.

As far as ABC, CBS and NBC are concerned, the trends aren't good. Their overall ratings are even lower than they were a year ago - when they were at an all-time low.For example, "Cheers" finished as the No. 3 show last season. This year it's a clear No. 1 - it's been at the top of the Nielsen list the first four weeks of the season - but its actual numbers are below last year's figures.

That's the way to be a success in the ratings war these days - lose less of your audience than the competition.

Another troubling trend for the networks is the fact that none of their new shows could be considered an unqualified hit. As a matter of fact, only a couple of them (NBC's "Fresh Prince of Beverly Hills," CBS' "The Trials of Rosie O'Neill) could be considered hits in any sense of the word - and they're only marginal successes.

Through the first four weeks of the 1989-90 season, the Big Three were averaging 71 percent of the total viewing audience in prime time. This year, that figure showed a big drop-off - all the way down to 65 percent.

For the first time ever, the Big Three averaged less than two-thirds of the viewers.

According to Nielsen figures, NBC's audience is down 13 percent, ABC's dropped by 9 percent, Fox's is off 6 percent and CBS - which already had the least to lose among the Big Three - experienced a 2 percent loss.

All of this has made for one of the closest network rating races in history - first-place NBC and third-place CBS are separated by only about 1 million homes.

One of the biggest reasons for the decline is the continued growth of cable. Through those same four weeks, basic cable channels increased their combined viewership by about 2 million homes.

PBS also put a dent in the Big Three, almost solely because of the amazing performance of "The Civil War" in the ratings.

Why are viewers switching away from the networks? One might think that people are tired of the same old shows and want something different. There's two problems with that theory:

1. A lot of the programming on basic cable is the same old shows - reruns of canceled network offerings.

2. Americans are overwhelmingly rejecting the more than 30 new shows ABC, CBS, NBC and Fox offer this fall.

A look down the top shows for the first four weeks of the season is a trip down memory lane - "Cheers," "60 Minutes," "The Cosby Show," "Roseanne," "A Different World," "Murphy Brown," "Designing Women," "America's Funniest Home Videos," "Empty Nest."

With the exception of "AFHV," now in its second season, these shows have been around for years.

Many of the new shows follow old formulas. But more daring, ground-breaking entries like "Cop Rock" and "Twin Peaks" haven't ignited any ratings fires -"Cop Rock" is shaping up as one the biggest bombs of the season.

Another reason for the decline in network share is Fox. FBC expanded to five nights of programming this fall, further splintering the audience.

Not that Fox hasn't suffered growing pains. As a matter of fact, those pains have been severe. With the exception of new episodes of "The Simpsons," this fall has been pretty much a disaster for Fox.

"Married . . . With Children" is a moderate hit at No. 34 for the season. "In Living Color" is next at No. 59. After that, all of Fox's shows are No. 70 or lower. As a matter of fact, Fox has the five lowest-rated shows and nine of the bottom 11.

The fourth network does have some built-in problems: It's got about 65 fewer affiliates than each of the Big Three (Fox reaches only 90 percent of the country) and many of those affiliates are relatively low-powered UHF stations (channels 14 and higher).

But the fact is that Fox stands to lose a bundle. Industry reports indicate that FBC is already having to broadcast free commercials to advertisers to make up for lower-than-expected ratings.

And perhaps most distressing for the networks is the fact that the future holds more of the same for them. Back when ABC, CBS and NBC were the only games in town, more than nine out of 10 television viewers in America tuned in.

This season, they're going to have trouble holding on to six out of 10. And before this falloff is finished, they may have settle for just one in every two.

Not that this is necessarily bad for the viewers, however. Just a few years ago, the hallmark of success for a network series was a 30 share (three viewers in 10 of those watching TV). So networks aimed for the lowest common denominator to grab the biggest audience - resulting in a lot of bland, boring programming.

Today, very few series reach that 30 share. So shows like "thirtysomething," "Twin Peaks" and "48 Hours" can survive with lower ratings, bringing a more diverse menu to the TV connoisseur.

And cable is coming up with more and more original series of its own, another boon for the viewer.

So while the networks may be worried, the TV viewer needn't to be. Granted, there's a lot of awful shows on TV. But there are also more good shows than ever before.

So with a bit of picking and choosing, you're likely to find something to suit your taste.