I'm not a pilot or engineer, but after reading every post on Techops and asking plenty of silly questions for 3 years, I feel comfortable saying that any landing on a large jet in which the Main landing gear touches down, then bounces up for 2-3 seconds before touching down again is an undesirable landing.
Your question, however, doesn't specify which venue of medium in which you've whitnessed this event. So here's a quick synopsis of typical situations that might lead one to believe that A330/340s touch down in the manner you describe.
-If you're basing your conclusion about the way in which these planes touch down on videos or in-person plane spotting, I submit that you've managed to get lucky in whitnessing some unusual touchdowns, since, as mentioned before, pilots around here have, time and again, expressed that a bounced landing or floating landing is far less desirable than a firm, but controled touchdown on the tarmac.
-The A330/340 landing gear trucks are somewhat unique in that the distance between the forward pair of wheels and rearward pair of wheels is somewhat greater than most other 4 wheel landing gear bogeys. In addition, these landing gear are angled dramatically such that the rear set of wheels hangs way lower than the front. The result is that any float or (ever so) slight bounce is magnified by the way the trucks tilt upon landing.
-If you're basing your consclusion about the way in which these planes touch down on in-cabin experience as a passenger, perhaps SlamClick, who's piloted the A330 has the answer. I recall, a while back, him mentioning that, even from the cockpit, the A330 has 3 distinct touchdown moments: one for the rearmost wheels of the MLG, one for the foremost wheels, and one for the nosegear--this likely due to the somewhat greater distance between the rear and front wheels of the MLG.
Just some thoughts...I doubt all, or indeed even most A330/A340s bounce in the manner you suggest.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.