Google script flextable span columns

I am trying to make an input box which spans the 3 columns of the table. From looking online it seems like I need to use the

setStyleAttribute

but I am not sure and cannot decipher the documentation. I know from here that

A flexible table that creates cells on demand. It can be jagged (that is, each row can contain a different number of cells) and individual cells can be set to span multiple rows or columns.