Match two columns in different sheets

Hello,

I have three work sheets with columns namley process code, process name, quantity produced per day. I need to create another sheet comprises of process code, process name and quantity produced monthly so that to sum up all the quantity produced for a particular process that appears in all 3 sheets. I have tried pivot table but my data also has blank spaces, hence vlookup is then also not returning correct values. Please please somebody help me on this issue! I'll be very very thankful. :)