How to define likelihood by myself in Pymc3

I want to get the sample result as an array. There are three parameters in my model, ‘amp’ ,‘rate’ and ‘E_start’. In v3, I use
backgroud_parameters=posterior3[‘amp’],posterior3[‘rate’],posterior3[‘E_start’]
If I want to accomplish the same result as v3 in v5, which function I should use ?