Which bike multitool should I get for a cannondale trail 3 2019? Which other tools will I need?
I wouldn’t get a multi tool.

All the tools you need for a modern bicycle (if you are not replacing bearings, crankset or cassette) are:


4mm hex key for:


seat post clamp
stem+handlebars
bottle holders
racks

5mm hex key for:


rim brakes brake lever clamps
derailleurs
saddle rail clamps

T25 torx for:


disc brake rotors

8mm hex key for:


pedals 

screwdriver for:


derailleur limit screws

spoke key for wheel spokes
tire levers
pump
chain tool


Get a set of proper hex keys and the T25 torx if you have disc brakes. Get tire levers, a small pump for the ride and a large pump for the workshop.

But on single day rides it doesn’t make sense to bring any of keys. You really only need the small pump, tire levers and a spare tube.

When I’m making saddle adjustments I take a 4mm hex key (and maybe a 5mm key if I’m also adjusting the fore-aft position) for the ride.

On multi-day travel I bring the 4mm and 5mm keys plus spare shifter cables and spare bottle holder screws. I also bring the spoke key and spare spokes (only makes sense if you know how to use it). A multi tool would have too many tools on it (which makes it heavier than the two dedicated keys) and they are usually short and fiddly.

If you have disc brakes and need/want a T25 Torx key I’d get this T25 Torx bit from Topeak which turns a 4mm allen key into a fully functional T25 Torx key.