In waltz, some teachers said body weight should be on beat ( 2nd beat) totally., some teachers said body weight should be between 2 feet ( make "A" shape ) shift weight when count 3, I really confused.

"Some teachers" can say about anything, really. It doesn't particularly mean they are right or wrong; they just have a different method of delivering the message. But, I would be confused if it was explained the way you summarize in your note. I was always taught that the hard emphasis or total body weight, if you will, is on the first step, because the weight lessens on the rise of 2 and 3. I think the second part of your note makes sense, as you definitely have to shift weight when completing count 3. Even on this site's learning center, it is made pretty easy to understand when it states "Strong accent on 1."
There may be some other approaches to this, and the long-time instructors on this site always provide valuable insight as to exactly what your instructor may be trying to say, or if there is something to that interpretation. Regardless of how you want to think of weight pressure and shift, I have always just thought about rise and fall. If you have that concept stuck in your mind, and your body is following it, I would think the weight shifts would occur the way they should.

Imagine a ruler that spans from 1 to 3 with equal amounts of distance between the numbers. Imagine the distances in terms of time between notes. We begin our first step on 1, and we have all of the time encapsulated in that beat to complete the step and transfer the majority of our weight over / onto the moving foot. We transfer some of our weight initially, then more weight, and finally most of it before the next step begins. The trailing foot will begin to skim over the floor to meet or pass the first foot (depending upon what figure is being executed) at the begining of count 2. As the second foot skims over the floor, it is providing balance by supporting some of your body weight. By the end of count 2, "most" of your body's weight should be on the new, supporting foot. The same process begins again at the beginning of count 3 and so on through your dance. And, depending upon the musicality of the song and the expression of the dance, there may be variations of the movement and weight change. Your body weight moves in time and across the distance between the begining of the notes. Try not to focus too much on the end of a note, as this might impede a smooth flight of the body across the floor and through time because no sooner do we arrive must we be ready to continue in our movement using the weight over the supporting foot to provide traction allowing the muscles to create movement. Rarely should all of your weight be over just one foot in ballroom and rarely, and particularly in the smooth dances, should we not have both feet in contact with the floor beneath us at most times. I find all of this to be a relatively easy concept to understand, but as of yet, I dont have it fully mastered.