Merge multiple columns into 1 in pandas

Question

i have a dataframe.

id    val    val2    val3
1      100    nan     nan
2      nan    200     nan
3      nan    nan     200
4      nan    500     nan

i want to merge columns val,val2, and val3 into 1. i.e my final dataframe should look like:

id    val
1     100
2     200
3     200
4     500