List Lookup multiple use in 1 form

I have a form which has 3 list lookup fields connecting to the same column in the same list.

do you mean they source listed entries from the same list and column or do you mean they are 'Connected to' (control's configuration option where they store values) to the same list and column?

if a value is selected in one of the dropdowns, that same value is not avaialble in the other two dropdowns for selection. Is this doable?

that would be possible only with a javascript.

Yes it is the same source list/column for the 3 dropdowns. 

If this requires javascript I don't suppose anyone would have an example that still uses the source list?

Thanks.

you may find an example here 

https://community.nintex.com/t5/Nintex-for-SharePoint/jquery-code-to-remove-add-options-to-dropdownlist-choice-field/m-p/14431