It looks like Block 3 is only a minor iteration with the following enhancements:
1. Updated HUD is obvious.
2. Updated MAWS is obvious.
3. The engine has changed. The obvious giveaway is the dark yellow primer on the engine housing and the chute housing. The exposed metallic cover around the aft section has a dulled color. The old, bright metallic cover would have been a better conductor of heat. Possibly, this section is now covered with thermal insulation, and the new protrusion at the front of the tail section allows more air for cooling. Finally, assuming the pic below shows the aircraft taking off, then it is unusual that the afterburner is not engaged.
The aircraft seems to be on dry thrust (compare with the picture of older aircraft taking off). Secondly, a constricted nozzle is used to attain a higher thrust. Block 3 seems to have an open nozzle and no afterburner during take off. This could be wrong analysis, and the aircraft might be landing, but the next picture in dafengcao's twitter post shows the aircraft facing the opposite direction, with the engine still releasing an exhaust plume, and the chute cover is open. So I will hazard a guess that the engine has a much higher power rating in Block 3. High enough, that the trailing flaps are lowered to reduce lift generated. To be honest it sounds unlikely, but the alternative is that the aircraft landed, then performed an about face, and paraded in front of the audience under its own power. Seems unlikely as well.
Given that the B model has been versioned 19-60x, it is likely that 20-30x will have the above enhancements, plus the drop in replacement LKF601E. Also, given what
@messiach has already said about different variants of thunder, possibly there will be 40x and 50x soon. One of them should be air superiority with the KLJ-7A AESA radar, and a ground strike version with delta wings. Of course, this is pure speculation on my part.
In any case, it is my humble prayer that May Allah the Almighty Guide us towards the right path, and make JF-17 a source of strength for Islam and Muslims in the whole world. Aameen.