Pierre
I believe you're 1/2 right...the levels shown on the screen ARE transmitted on 1013.2 AND converted to the same datum (below TA) which will be whatever is input into your radar display/system.
Some use 1013 (scary) and some input their local QNH. If QNH is input, and we are talking at or below TA (which in your example of 3000' you are), then unlike in your example, the mode C readout will be relevant, i.e. 2800ft descending will be below unknown traffic @ 3000ft...and better still, this will be based on 1013.2 set in the encoding altimeter and not dependent on the pressure set by the pilot on the sub-scale. WYSIWYG