What happen if i have game pass and gold, but I want the game pass ultimate on pc?

It depends, if you also want to have game pass for pc I would cancel gold and xbox game pass and subscribe to ultimate. But, if you don't want to play on pc, I'd stick with yearly gold and monthly xbox game pass.

and as far as i know, if you buy more gold or gamepass before you switch to ultimate, you get more ultimate at the end.

so all will be switched to ultimate.

The game pass and gold you have now will be converted into Ultimate since Ultimate includes those. You would actually receive 9 months of Ultimate since you have 3 months of each.

you would get 9 months of ultimate

Unfortunately you would have to purchase gold again if that is what you want. Before purchasing the promotion, you are warned that whatever gold or regular game pass you have on your account will be converted to Ultimate Game Pass and cannot be converted
back.

Xbox Game Pass Ultimate:

Use your active Xbox Game Pass Ultimate membership to play games on Xbox One and Windows 10 PC. Xbox Game Pass Ultimate includes Xbox Game Pass for Console, Xbox Game Pass for PC, Xbox Live Gold and additional benefits. Note: Purchase of Xbox Game Pass Ultimate
automatically upgrades your existing Xbox Live Gold or Xbox Game Pass membership(s) to Xbox Game Pass Ultimate at a conversion ratio each based on days remaining. Learn how this works at www.xbox.com/gamepass. Future code redemptions are also subject to a
conversion ratio. Conversion ratio subject to change. All conversions to Xbox Game Pass Ultimate are final: Once converted, you cannot re-convert your Xbox Game Pass Ultimate membership back to a previous membership. Maximum 36 months of redeemed Xbox Game
Pass Ultimate per account at a time, including any converted time from Xbox Live Gold and/or Xbox Game Pass (for PC or console).