3, 12, 21, 30, 33, 102

We observe that those are all multiples of 3.  
Now we'll find all the multiples of 3 to find
out why those are different from the other multiples of 3.

Here are all the multiples of 3 less than 200

3, 6, 9, 12, 15, 18, 21, 24, 27, 30
33, 36, 39, 42, 45, 48, 51, 54, 57, 60
63, 66, 69, 72, 75, 78, 81, 84, 87, 90
93, 96, 99, 102, 105, 108, 111, 114, 117, 120
123, 126, 129, 132, 135, 138, 141, 144, 147, 150
153, 156, 159, 162, 165, 168, 171, 174, 177, 180
183, 186, 189, 192, 195, 198

I notice that all the others have digits which are 
greater than 3, so this is the sequence of all multiples
of 3 which have no digits larger than 3.

So the sequence goes like this

3, 12, 21, 30, 33, 102, 111, 120, 123,
132, 201, 210, 213, 222, 231, 300, 303, 312, 321,
330, 333, 1002, 1011, etc.

You just wanted the next one after 102 which is 111.

Edwin