When do you play these notes? (Scriabin Etude Op 8 No 12)

[mathandmusic]

Disregard key sigs in this example. The L.hand plays triplets on beat 3 and 4. That's straight forward.

Does the right hand play the C+E at the start of the 4th beat or in between the last note of the 3rd triplet and the first note of the 4th triplet?

When do I play the A-octave on the 4th beat: the same time I play the last C-octave on the 4th triplet or halfway through the 4th beat?

What does that "3" stand for?

[j_menz]

The three means that the treble staff notes are now also a triplet.

The CE are played at the same time as  the first note of the last triplet. The A octave is played at the same time as the last of the triplet notes in the bass.

[iansinclair]

to further clarify... note also that in the right hand, the octave G, second half of the third beat, is tied to the octave G, first TWO THIRDS of the fourth beat -- as j_menz says, that "3" indicates that that last beat, right hand, subdivided into 3; two of the three on the octave G held from the previous beat, the last third on the octave A.

The right hand C and E happen on the fourth beat,at the same time as the left hand B, and hold through the beat.

The left hand octave C happens at the same time as the right hand octave A