I suggest focusing on ease of maintenance and flexibility, you can likely get transaction data csv's from each brokerage account, but they might not all report the same way. Rather than fight with the output they provide each time you load it, I suggest one worksheet per brokerage output where you dump the csv as-is. Depending on how they report you may want another worksheet per brokerage output that you use to standardize the transaction data.
If you want to track historical exchange rates and stock prices for your positions I would suggest a separate sheet for those, if using Google Sheets you can make use of their handy GOOGLEFINANCE() function to get that data.
Then I'd bring the data together in summary sheets. I favor this many-sheet approach because it makes for easy refreshing and preserves original details that I may not have used initially. It also allows multiple types of analysis from the same workbook without repeating source data or having one giant worksheet.
Converting everything to one currency first would get you to your bottom line, but it would muddle performance reporting/evaluation due to changes in exchange rates. This may or may not be a concern to you, but storing the exchange rate at time of trades will give you added flexibility.
I see a lot of example sheets and how-to's, here are a couple that may give you some ideas on how you could do this in Google Sheets (Plenty of Excel examples too, it's mostly basic spreadsheet functionality so any tool should be able to get you what you need):
Stock Portfolio Tracking Spreadsheet - Google Docs & Spreadsheets
Stock portfolio tracking with Google Sheets