if I understand you correctly, you have a series of 'parts' which are made of (up to) 3 'components', (I used different wording just so I am clear).
I would use 2 tables. tblpart and tblcomponent, using partid as the foreign keyin the component table.
This way, you can have as few or as many components for each part, without having redundant data fields.
to get your sum total, you would query the component table using the part id as a filter.
if this doesn't seem straightforward, here's an example. - at least then we'll both know if I understand your issue