Is it true?

Sort of. Sinaiticus and Vaticanus are certainly used, and given much weight because of their age, but they are not used exclusively, but rather in addition to the other texts (which are very numerous).

As to "3000 disagreements", it is my opinion that that number is used sensationally. Most of these are very minor, similar to the differences found in any two manuscripts from any text line. Differences should not disqualify a text from consideration, in fact differences often provide clues as to determining how and why various readings originated, what reading is more likely to be correct, etc. I think it is very important to consider all the evidence.

One last thought about "differences". If differences between two manuscripts disqualifies them, then we need to throw out ALL manuscripts and ALL Bibles. For example, Scrivener's edition of the TR contains approx 250 "differences" from Stephanus' edition of the TR - yet they are both "the TR": this is comparing different editions of the *same text* and still we find this many differences. Instead of throwing up our hands in resignation, or throwing out the texts because of "corruption", it is far more valuable and beneficial to take all the evidence, and employ quality textual criticism. It is more work, yes. But it must be done.