The error you're seeing is because the third switch isn't connecting to the Dashboard - hence the orange LED - and so isn't seen as being part of the stack. First, if you haven't already, make sure all the stacking cables are pushed home properly in all the switches.
IF that doesn't change things, take the switch with the orange LED out of the stack both physically and within the Dashboard. See here for the Dashboard part, under Managing Stack Members, https://documentation.meraki.com/MS/Stacking/Switch_Stacks.
Then try and get the switch working as a standalone device, e.g. do a factory reset etc. If you can't get it working then log a ticket with support as its possible the switch has 'gone bad', and need replacement. If you do get it working as a standalone you should then be able to add it back into the stack.