What are you trying to achieve? For example, do you have an Excel file with a ‘full name’-column (string) and do you want to separate it into ‘first name’, ‘middle name’ and ‘last name’ after importing it?
The closer to the source this datafield gets separated into three different columns, the better. Someone or some party is sending you this data. It happens that the supplier does a concatenate of the field to merge it into one field becase they think this is easier for you. Just asking is sometimes enough to get it handed to you with the three columns you want. That is the ideal solution.
Preprocessing in Excel is an option. Excel has function that do just that, and do it well. But you will need to do it manually every time. If this is a dataload that you will only do once, or a minimal number of times, this is good option.
The last option, which I think you are likely to use, is to have your Excel-import get followed by an importmapping and using a conversion-microflow:
This option gives you the most control over the splitting of one cell to three separate fields.