That sounds a bit challenging. My best advice would be to start very slowly on just D doing the tut tut tut thing. Then slowly doing the same thing on E. After that perhaps alternating notes. The G is a tough one, but once you have the D and E under control it may be a bit easier.
Another exercise might be to play long short long short (tah tut tah tut) over and over to get a feel for the difference in tonguing.
Disclaimer: I used to be a pretty good clarinet player 20 some years ago, but I have let my skills slip due to a major focus on sax.