Dashed line with more dashes: intermittent stream

The standard symbol for a stream on topographic maps is a blue line.

The standard symbol for an intermittent stream on topographic maps is a a blue line, with long segments interrupted regularly with a series of three dots. I can create a dashed line interrupted by two dots, but there aren't enough boxes for me to specify the third dot.

Any suggestions on the best way to accomplish this?

Thanks.