What's the use of the tax1, tax2 and tax3 columns

[joebordes]

These are the three default tax types you can find in Settings > Tax Configuration
If you add another tax there you will get another column (tax4) in the vtiger_inventoryproductrel table
The S&H values and taxes are stored in another table (vtiger_inventoryshippingrel and vtiger_shippingtaxinfo)