was this show successful?

Well, they did two out of the three books, so it was successful to a degree.

No, is the answer. It was a complete failure. As drama, as sci-fi, as an adaptation of the novels, as television. As art, as popular entertainment. I was part of the target audience when it was broadcast, and I remember being bored to tears by it and preferring The A-Team. And I loved 2001; I could sit down and watch something slow and engrossing. The Tripods was just slow. Nothing happened. The first series could have been shortened to two fifty-minute episodes without losing anything. And although it was expensive to make, there weren't many Tripods. The show wasn't called Trek Through Northern France with Weedy Kids; it was called The Tripods, and that's why I watched it. I was sorely disappointed.

It was axed by new BBC controller Michael Grade, who also cancelled Dr Who at much the same time. Because they were both rubbish. They were rubbish. As far as I can tell it has never been repeated on British terrestrial TV, and so for people my age it's just a memory of a show that had been on long ago and was cancelled before it finished. There's a good article about it here:
http://www.gnelson.demon.co.uk/tripage/making6.html

It's notorious for being the last gasps of old-school British TV sci-fi, along with Star Cops, which was also a dismal waste of money. Red Dwarf absolutely trounced both shows; it was funny, the sci-fi was smarter, and when I was at school it was hip. The Tripods was years behind the curve.

The title sequence was ace though:
http://www.youtube.com/watch?v=3AmykynKhMw

According to http://www.super8web.de/tripods/staffel3.html (It seems to be a fansite but I see no reason why it shouldn't be reliable. It's in German so unless you speak German you either have to take my word for the contents or use a translator.) it was a very huge success. According to the link the viewing figures were around 10 million viewers in the UK throughout all of its run even though there were only 8 million kids in the UK at that time. According to the side that number is similiar to the one that Star Trek Next Generation had when it started in the USA.

Apparently Tripods got axed because of the high production costs that would have been even higher in season 3. The BBC was afraid that the viewing figures would go down with the Sci-Fi boom coming from Star Wars subsiding in the Mid-80s. (That was before it went up again with Star Trek Next Generation). BBC had 2 Sci-Fi series at that Time - Tripods and Dr. Who and one was supposed to be axed. So they axed Tripod even though it was more successful than Dr. Who - but Dr. Who was much cheaper. At least that's what the linked side says.