Sisodia accused them of defaming him through "offending publications" based on "cooked up" allegations and sought their "complete, express, unqualified and unambiguous apology".
Sisodia, in his three-page notice, termed the "offending publications" the statement made by the three in various media houses as a "concerted strategy" and devised by the BJP think-tank for "narrow political gains".
The statement, the notice reads, were "not merely defamatory and motivated, but also false, baseless, incorrect, unverified and misleading"
Responding to the legal notice on Twitter, Tiwari said he refuses to "bow" to Sisodia's "pressure tactics" and will prove their alleged corruption.
He said they will go to the Lokayukt and appeal to courts in the matter.
"We will prove your corruption ... BJP stands by the people of Delhi and will recover this enormous sum of taxpayers money," Tiwari tweeted.
Singh too reacted on Twitter and said legal threats cannot triumph against their truth.
"Try no matter what, these legal threats cannot triumph against our Truth. We are determined to fight against corruption. Our commitment for people of Delhi will not be deterred; try no matter what," Singh tweeted.
Tiwari, the Delhi BJP chief, had claimed on Monday that there is a "scam" of Rs 2,000 crore in the construction of classrooms for Delhi government schools.
Citing an RTI reply, Tiwari had claimed that the AAP government constructed 12,782 classrooms at a cost of Rs 2,892 crore and this could have been done by spending just Rs 800 crore.
( With inputs from IANS )