To figure out the initial conditions, simply write out the ternary strings for n=1 and n=2:
That should be sufficient for some initial conditions. I guess you could also do it for n=0, but there are 0 of length 0.
From here, I would suggest writing out for maybe n=3 and n=4. Beyond that, you may want a computer to spit out such things. Then you could look for a pattern and find the recurrence. That's just a brute force sort of idea.
For now, it's all I've got. We may need to think about this more mathematically, that is, what is the equation for the number of unrestricted ternary strings? Then, what is the mathematical restriction we wish to impose?