How do I autofill 3 login fields for a website, please?

Hi @oatbean

I've taken a look at the website in question, you can save an item in 1Password to fill all 3 fields with the following steps:
1. Fill the information into all 3 fields. Do not click "Ingresar" yet.
2. Right-click on the 1Password icon in your browser's toolbar and select Save Login. Alternatively you can right-click on the field and select 1Password Password Manager > Save Login.
3. Choose to create a new item or update an existing one.
4. Save the item.

When viewing the details for the item or opening it in Edit mode, you should see a new section labelled "Saved on web.inss.gob.ni":

Moving forward selecting this item using the inline menu suggestion from either NSS or CONTRASEÑA will fill all 3 fields (Though I've noticed the inline menu is not displayed when PATRONAL is selected):

Give that a try and let me know how it goes.