I have a 3x10 speed MTB. Will it be a upgrade or a downgrade to move towards a 1x10 and will I be able to easily ride above 60km per hour as I can with the 3rd gear in front?
1x drivetrains are a newer concept made possible by 11 speed rear derailleurs that can support an increased spread of sprocket sizes in the cassette. Most people would regard 1x as a upgrade over 3x. There are definite advantages: lighter, only one shifter, no cross-chaining. less chain slap and better retention.
You do have to be careful about gear ratio range and steps between ratios, especially with 1x10 as opposed to 1x11. With a single chainring you need a wider spread of sprocket sizes to get the overall ratio range, which gives you bigger steps between ratios.
You can use a an online gear ratio calculator tool to compare the ratios you have in your current 3x10 setup and what you can get in a 1x10 setup, and also see the speed for given cadences.