Since you found that there are few similarities in the amino acid sequences for alpha amylase in the three organisms, how do we account for them being functionally similar? We need to take one more step and examine the three dimensional structure of the enzymes. You can use tools on the NCBI website for this as well.
Open the NCBI main page. Click on Domains and Structure on the left hand menu bar, and then select Conserved Domain Database (CDD) under the resource tab.