Tory MSPs Oliver Mundell, Douglas Lumsden and Sharon Dowey have all swung behind the favourite to be the next prime minister.
Writing in Holyrood magazine, Mr Lumsden said the foreign secretary “more than most understands the value of the Union”, having been born in Oxford and partly brought up in Paisley.
He said: “Liz will work constructively with the Scottish Government, but will not be intimidated by the grievance politics of the SNP.”
The north-east MSP added: “Ultimately this contest is about the direction of our country and the course we chart to navigate many challenges – a Government focused on delivery, protecting Scotland's place in the UK and championing the UK's place in the world underpinned by our values of liberalism, democracy and enterprise.
“This is what a Liz Truss premiership will deliver and that's why I'm backing her.”
Ms Dowey said she was backing Ms Truss “after careful consideration”.
It comes after a string of endorsements for Rishi Sunak, the former chancellor and Ms Truss’s leadership rival.
He has secured the support of high-profile figures including former Scottish Tory leader Ruth Davidson, as well as more than half a dozen MSPs.
However, many have yet to declare.
Scottish Tory MPs Andrew Bowie and John Lamont have both backed Mr Sunak, although others, such as Scottish Secretary Alister Jack and Scottish leader Douglas Ross, will not publicly support either candidate.
Writing in the Daily Telegraph, Ms Davidson insisted: “For his ability, intellect, leadership, decision-making, work ethic and track record of making the right calls to steer our economy, my vote is for Rishi Sunak.”
Jackson Carlaw, another former leader in Scotland, has also come out in support of the ex-chancellor.
Polls previously indicated Ms Truss is the favourite among UK Conservative Party members.
Around 160,000 members will decide who wins the keys to Number 10, with a result announced on September 5.
Both candidates have held online calls with MSPs in a bid to secure support.
Former Tory MP Ross Thomson, who was Boris Johnson’s Scottish campaign manager during the 2019 leadership election, said he was backing Ms Truss.