I think you can but double check the watts per foot. The 6" tape should be about 12watts per foot and 3" tape should be about 4 watts per foot, so if all strips are 1 foot each then you should be good as long as the thermostat is rated for a higher wattage.
6" one foot each...3strips x12watts =36 watts
3" one foot each...6strips x4watt =24 watts
24+36=60 watts total
I would say yes it will work as long as the thermostat is rated higher than 60 watts.
Yes..what Deb Said