I cannot hit with Paul Goldschmidt to save my life

I just can't hit with him, so do I sell him and buy another third baseman like Rizzo or do I put Gallo at First, put Mathews at Left and Longo at third?

You won't know if you can hit with a player until you try yourself. Choose one of those options, and if it's good, keep it, and if not, change it up.