Mathematically characterise 3-series, and their interaction

Dear smart people,

Can you give me some guidance on a characterization problem. I want to characterize behaviour of these 3 lines, they cross each other and then diverge eventually.

Where should I start?



Not a robit
I haven't worked much with time series, so I will see what vague help I can provide. How often were data collected in these series, it seems like perhaps weekly.