Since you are making 2 separate bookings there is no guaranteed safe time . 3.5 hours should be plenty of time to get across and check in for the EK flight however if you are delayed and miss it, neither airline will be responsible for getting you home from Bangkok

what you might want to check is what the change fees/penalties are for no show in case the worst happens ; if it is a completely inflexible booking, I would probably err on earlier still but if the penalty should the onward flight isnt too bad, then stick with the 3:30 connection