What do you say to share your deeds after you have donated something? I'm wondering do you say "sadu, sadu, sadu" in theravada buddhism, after you have made donation and want to share your good deeds with others.
No. Sadhu means something like 'well said'.

I think you're referring to sharing of merits:

Patti Dāna

Idaṃ me ñātinaṃ hotu

(May this be for my relatives.)

Sukhitā hontu ñātayo

(May my relatives be happy!)

If you want to hear the chant in action (and chant along):

http://chantpali.org/closing.html#patti_dana

Another option:
“Idaṃ me  puññaṃ nibbānassa paccayo hotu.” 
(May these merits of mine be conducive to my attainment of Nibbana)

Addition:
I think it's also noteworthy that there is not one correct way to share merits. You can use your own words, in your own language, in English, Pali. Whatever feels comfortable to you. I think that an important part of sharing merits is to really connect to the meaning of the words you're saying. Otherwise it's just 'blabla'.