On a flow I have for creating tasks I hadn't set up a due date, but using this expression seems to work
I'm using the created date trigger triggerBody()?['Created'] as the basis for when it should be due so Monday would be due Thursday, Tue due Friday, Wednesday spans the weekend so we need to add 2 days, same for Wed/Thur/Frid, Saturday due Wednesday and Sunday also due Wednesday.
addDays(triggerBody()?['Created'],
if(equals(formatDateTime(triggerBody()?['Created'], 'ddd'), 'Mon'), 3,
if(equals(formatDateTime(triggerBody()?['Created'], 'ddd'), 'Tue'), 3,
if(equals(formatDateTime(triggerBody()?['Created'], 'ddd'), 'Wed'), 5,
if(equals(formatDateTime(triggerBody()?['Created'], 'ddd'), 'Thu'), 5,
if(equals(formatDateTime(triggerBody()?['Created'], 'ddd'), 'Fri'), 5,
if(equals(formatDateTime(triggerBody()?['Created'], 'ddd'), 'Sat'), 4,
if(equals(formatDateTime(triggerBody()?['Created'], 'ddd'), 'Sun'), 3, 0)
)
)
)
)
)
), 'yyyy-MM-ddTHH:mm:ssZ')
Hope that's of some use