Made for 1,75mm filaments
If you want to use it with 3mm you can easly resize it to 171-172%

Flat part goes under the filament and the 'head' holds one or two lines (depending on version)
Small Filament Clip is without flat part, may be better for smaller spools where you don't have so much space

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Printed with 0.4mm nozzle with 0.1mm layer height

If it will hold filament too weak you can try printing it with slightly smaller size (like 98-99%) or with little more flow