7 new planets possible 3 could support life.
How do they know it's only 3?
Those three are within the habitable zone of the star. This means their surface temperature could support liquid water. However, this is really a best guess about whether they are suitable for life it or not. There are too many other variables. Similarly we can't really say the others could not support life.
No-one. They worked it out themselves. This is what science is.
We are supposed to believe them because they're scientist's as they know everything or do they?
No. We don't have to believe them. If we have the training and knowledge, we can look at the same evidence and decide for ourselves if we come to the same conclusion. This is why science is peer reviewed and papers are published; there is a huge amount of scrutiny of results, especially controversial ones. The problem comes when you don't have the knowledge to do this for yourself. Then you don't have to believe the scientists, but you do have to believe in the process.
Attempts to discredit science by people who disagree with some of its conclusions are some of the more serious and insidious threats to civilization, both in the short-term and the long-term.
Are these the same 7 planets reported in 1993 and dismissed by US scientists.
I have to admit I don't know what you are referring to here. But, it seems almost certain they are not, as these seven are recently discovered by methods that were not available in 1993.
will we ever know the truth?
There is often no 'truth' in science, especially in astronomy, where all information comes from little wisps of light that are thousands of years old. If people actually understood how much we have worked out from so little, they would worship astronomers as the gods they really are.
Science is constantly revising or rejecting ideas that were previously consider completely correct. Often there is uncertainty about specific numbers or there may be processes with something that is generally understood that are themselves not.
People who are 'anti-science' always use this uncertainty to criticize and delegitimise it. In fact this flexibility is the strength of science. It can, and does, change itself. The lack of dogma is one of the things that makes science better than the alternatives.