The Brazilian is being watched by the trio of clubs, but while there has been no concrete offer for him, he is not prepared to upset his current side by stating what he wants to do next term.
Diego has been linked with Madrid and Juve on several occasions and while Manchester City's interest is new, their spending power could well test the other clubs.
"Juventus, Real Madrid and Manchester City are watching me," sportmediaset quoted the playmaker as saying.
"In May, I will decide what to do.
"Now I am only thinking about the next few games for Werder. I will talk to the club when it is the right time."
InterestWith his next move drawing much speculation, Diego has sought to put any distractions to one side as his father will handle any offers or negotiations.
"My father is focussed on that side of things, while I am concentrating on playing," he told Spox ahead of Werder's Uefa Cup encounter with AC Milan.
"If a club is interested in me then they have to first speak to Werder.
"At the moment, I know of offers but am only focussed on Werder.
"I am very happy here and my father can also see that."