If you mean: does a Cleric 3/Wizard 3/Mystic Theurge 10/Sorcerer 1/Ultimate Magus 10 have the spellcasting of a 20th lvl wizard: yes.
(UM spellcasting progression is 7/10 for highest class)
If you mean: does the Mystic Theurge count as a spellcasting class, implying that both cleric and wizard would gain additional spellcasting levels: no.
If you mean: does a Cleric 3/Wizard 3/Mystic Theurge 10/Sorcerer 1/Ultimate Magus 1 have the spellcasting of a 15th lvl Sorcerer (implying the Mystic Theurge adds lvls to the sorcerer class as well): no
@Dandu: Powerwise, it indeed looks like a horrible build. On the other hand, if you go for low level spellslots in abundance (and you have the stats for it), it looks interesting. Beguiler or something like that might be a better choice than sorcerer, however.