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:
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):
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'.
You can speak out loud or silently in your mind. What is important is that the intention to share/benefit yourself or others is present.
Here is a quote on the transfer of merit. Its discussed what the benefits are and how the transfer is done. The method can be used for living beings as well. The quote is from the book "What Buddhists Believe" by Ven. K. Sri Dhammananda, p. 409-410:
The Significance of Transference of Merits to the Departed
"ACCORDING to Buddhism, good deeds or ‘acts of merit’ bring happiness to the doer both in this world and in the hereafter. Acts of merit are also believed to lead towards the final goal of everlasting happiness. The acts of merit can be performed through body, speech or mind. Every good deed produces ‘merit’ (store of positive spiritual well being) which accumulates to the ‘credit’ of the doer. Buddhism also teaches that the acquired merit can be transferred to others; it can be shared vicariously with others. In other words, the merit is ‘transferable’ and so can be shared with other persons. The persons who receive the merit can be either living or departed ones.
The method for transferring merits is quite simple. First some good deeds are performed. The doer of the good deeds has merely to wish that the merit gained accrues to someone in particular. This wish can be purely mental or it can be accompanied by an expression of words.
The wish could be made with the beneficiary being aware of it. When the beneficiary is aware of the act or wish, then a mutual ‘rejoicing in’ merit takes place. Here the beneficiary becomes a participant of the original deed by associating him or herself with the deed done. If the beneficiary identifies him or herself with both the deed and the doer, he or she can sometimes acquire even greater merit than the original doer, either because the elation is greater or because the appreciation of the value of the deed is based on an understanding of Dharma. Buddhist texts contain several stories of such instances.
The ‘joy of transference of merits’ can also take place with or without the knowledge of the doer of the meritorious act. All that is necessary is for the beneficiary to feel gladness in the heart when he or she becomes aware of the good deed. If one wishes, one can express joy by saying ‘sadhu’ which means ‘well done’. What is being done is creating a kind of mental or verbal applause. In order to share the good deed done by another, what is important is that there must be actual approval of the deed and joy arising in the beneficiary’s heart. Even if so desired, the doer of a good deed cannot prevent another’s ‘rejoicing in the merit’ because he or she has no power over another’s thoughts. According to the Buddha, in all actions, thought is what really matters. Transference is primarily an act of the mind.
To transfer merit does not mean that a person is deprived of the merit originally acquired from his or her good deed. On the contrary, the very act of ‘transference’ is a good deed in itself and hence enhances the merit already earned".