where is your continent of origin?

if you start from europe/asia.
one way you could do this, is have this as your last segment and to forfeit the euro-asia intercont.
then you will have to arrange surface transport for PEK-asia origin, or Moscow-euro origin. e.g.

but talk to them, perhaps they wd allow this.

IMHO, if u r only doing 3 continent, it make make more sense to buy simple 1-way and visit-continent pass