I suppose like everybody I need to ask the question.

Mrs DJ flying into T1 on Qatar. Flying out of T3 on Singapore. Separate tickets!

So land in T1, proceed to T3. Go to Singapore Airlines transit desk? They get the bags and put them on to the new flight? Seems very easy if that is it.

Confirm she she does not have to clear immigration. Collect bags, change terminal and the re-clear security and check in for new flight?