I like to point out that I am just getting started out on the a320, and just looking for some insight on usage of speed breaks.
I read somewhere that they should not be used in managed descent, but only in open descent?
Not sure if this is true, but i would like to understand why, if it is.

And lastly, any other times I shouldn't use them? I read they should be used only up till flap 3, correct?
Also(silly ques), does the same concept of using them apply to the 319, 321 as well?

Thanks n advance!

The use of speedbrake is not prohibited in managed descent. However, if you are on or below the FMGC calculated descent path, and use speedbrake, the ATHR will increase thrust, leading to the clearly unwanted situation of using power vs. speedbrake.

Sometimes, if you are significantly above the FMGC calculated descent path (or your speed is above the calculated descent speed), you will get a message "MORE DRAG" on the PFD, which tells you that the FMGC doesn't think that it can make the calculated descent path without the use of half speedbrake. This is the only time you would generally use speedbrake in managed descent.

The speedbrake is inhibited in CONF FULL, so you are limited to using them down to CONF3. It is "Not recommended" in our company SOP to use speedbrake below 1000'AAL.

The speed brake concept is broadly the same on the 319 and 321, though the A320 is slightly different in that speedbrake deflection with the autopilot on is limited to half of the maximum deflection.

May I suggest the following regarding use of the speed brake and high speed descents.

Above 315kts they can take quite a while (20 sec from memory). If you travelling faster than 320 kts, I strongly suggest you control the deflection commensurate with AP pitch to avoid an Overspeed. Too fast and the 'G' pitch authority may not be enough.