How are different sport climbing disciplines combined?

The three results will be combined by considering only the rank in each discipline and multiplying all results, see here. The lowest combined rank wins.

There will be a qualification and then finals. 20 athletes of each sex (20 men and 20 women) will be in the qualification as far as I know and the best 6 will go to the finals.

Speed will be first, than bouldering and finally lead.

How does multiplying the ranks work and what does it mean?

To give an example, consider a speed climber who climbs reaches the top in speed with the lowest time (1st rank in speed) but who is not among the top group in either lead (say 15) or bouldering (say 18). He/she will have a combined score of 1*15*18=270.

This speed climber will be better than an mediocre guy who gets the 10th place in each discipline (10*10*10=1000). This shows that specialists are favored compared to all-discipline people (if one would look at the average rank, the all-discipline guy would win with 10 compared to (1+15+18)/3=11.33 ).

However, the overall winner will probably be someone who is skilled in bouldering and lead (speed needs a very different skill set compared to bouldering/lead). To see that, think of someone is last in speed, but first in bouldering and third in lead. This guy will have a score of just 1*3*20=60, much lower than the above described speed climber (even if he/she would be say 10th in bouldering or lead).