The trio were relieved of their duties as Boris Johnson wielded the axe in his first shake up since the general election win.
Now former Northern Ireland secretary Mr Smith was the first big name to go, followed by Ms McVey then Mrs Leadsom.
Chris Skidmore was also ousted from his role early on.
Ms Leadsom tweeted: “It has been a real privilege to serve in Government for the last six years, and in @beisgovuk for six months. I now look forward to focusing on my constituents and on my 20+ year campaign to see every baby get the best start in life.”
Number 10 previously said new appointments would reward MPs who have worked to “deliver the change people voted for last year”.
“The Prime Minister wants this reshuffle to set the foundations for government now and in the future,” said a Number 10 source.
“He wants to promote a generation of talent that will be promoted further in the coming years.
“He will reward those MPs who have worked hard to deliver on this Government’s priorities to level up the whole country and deliver the change people voted for last year.”