Question 525519
Let {{{t}}} = Jill's time to clean backyard
given:
{{{ t/2 }}} = Mike's time to clean a backyard
{{{ t - 1 = 3 }}} = John's time to clean a backyard
Let {{{ q }}} = time with all 3 working
----------------
{{{ t - 1 = 3 }}}
{{{ t = 4 }}}
Add their rates of working to get rate working together
{{{ 1/t + 1/(t/2) + 1/(t-1) = 1/q }}}
{{{ 1/4 + 1/2 + 1/3 = 1/q }}}
Multiply both sides by {{{12q }}}
{{{ 3q + 6q + 4q = 12 }}}
{{{ 13q = 12 }}}
{{{ q = 12/13 }}} 
It will take all three 12/13 hr
( this is about 55 min and 23 sec )