It is possible to create a line chart with 3 lines.
For example, in the NSQL the data would like:
SELECT @SELECT:DIM:USER_DEF:IMPLIED:x:fin.unique_id:unique_id@,
@SELECT:DIM_PROP:USER_DEF:IMPLIED:x:fin.period:period@,
@SELECT:DIM_PROP:USER_DEF:IMPLIED:x:fin.period_name:period_name@,
@SELECT:DIM_PROP:USER_DEF:IMPLIED:x:fin.f_amt:f_amt@,
@SELECT:DIM_PROP:USER_DEF:IMPLIED:x:fin.b_amt:b_amt@,
@SELECT:DIM_PROP:USER_DEF:IMPLIED:x:fin.a_amt:a_amt@
When you define the Line Chart portlet, you specify the Source Data that displays on the Primary Axis Metric, just push over the required attributes. In the above example, I have pushed over f_amt (forecast cost), b_amt (budget cost), a_amt (actual cost).
In the Options "Entire Chart", you then define how each of these are displayed which includes Line (as well as Area and Column).