For prototyping, I would suggest to try TO220 through hole 3V3 logical level Vgs(th) IRL540N N-channel power MOSFET: infineon.com/cms/en/product/power/mosfet/…. It might be risky to use SMD device.

What does xyz mean? What do you think it means? What do you think would happen of xyz was very small/large? Also it is strongly recommended to add a schematic so that we can see HOW you will be using the MOSFET. Your "What does xyz mean?" tells me you're not that experienced so in order to give sound advice I need to see how you're going to use the MOSFET. For that, include a schematic. Some xyz are more/less important depending on how you use the MOSFET.

@tlfong01 thanks for your reply. I will try that one for prototyping.

@Bimpelrekkie thanks again for your reply. I will add a schematic shortly if it helps to clarify the question

An N channel MOSFET is an inappropriate choice for controlling a 12 volt power rail unless you are prepared to add a voltage boost circuit. It's better to control the 0 volt rail with an N channel MOSFET.

I agree with starting with a commonly used MOSFET like the IRL540. Chances are that it will do the job. As a beginner you're over-worrying about selecting the right component while experienced designers (like me) know that many commonly used devices can do the job (assuming you're not doing anything extraordinary). Also beginners often over-worry about the components but then use the wrong circuit!

What Andy mentions is a extremely common trap for beginners that 9 out 10 fall into (and then come here and ask why it doesn't work). And that's why we need to see a schematic. Are you making a source follower or not?

How are you going to solder the LEDs? What are you going to mount them on?

#OYPS. I usually strongly recommend power LED lamp newbies, before starting to do any circuit design, to read the following: (1) One Watt LED - Components 101 2018mar17: components101.com/diodes/1-watt-led. After reading this short, newbie friendly tutorial, you might like to let me know if there was at least one important thing that you didn't know that you didn't know. Happy reading. Cheers.

#OYPS, you might also like to watch how other guys are playing with power LEDs. (2) Power LED's - Simplest Light With Constant-current Circuit - dan, Monkeylectric, instructables.com/… Happy reading. Cheers.

@tlfong01 thanks for your useful articles.

@Bimpelrekkie I have added an example schematic to the question. I hope this helps to clarify things.

@Andyaka they will be reflow soldered on to a pcb.

Are you hoping to drive the LEDs at around 100 mA @OYPS

@Andyaka no, i am pretty sure they will draw more than that. Maximum of 700mA. Why?

Well, 700 mA through an 18 ohm resistor drops 12.6 volts and that's more voltage than the supply. Whereas 100 mA through an 18 ohm resistor drops 1.8 volts leaving 10.2 across 3x LEDs or 3.4 volts each. You also need to specify which LED specifically you are using.

And, 700 mA is the absolute maximum rating where they may get destroyed if you go a little bit more. You should aim for no more than 500 mA.

@Andyaka You are right. The schematic had the wrong Resistor value. It should be 1,8Ohm as calculated via this amplifiedparts.com/tech-articles/led-parallel-series-calcula‌​tor. The LEDs datasheet is listed the question. I can increase the value a bit to limit the current to 500mA. Thanks for pointing this out.

You didn't try to use a source follower which is good. You're switching the - side with an NMOS also good. Re-calculate that resistor and also how much power it is going to dissipate! It will need to be able to handle that or you will get a burned resistor.

You say that Vth should be smaller than your logic level. Good! That's right. But also it must be much smaller than your logic level. A common pitfall people fall into is thinking that Vth is the voltage at which the FET can be considered fully on, when that's not true at all. It's the voltage at which it just barely begins to conduct, and depending on the FET you may need to be as much as several times Vth to turn it fully on. You want a FET with an Rdson specified at a Vgs of your logic level or less, and specified to be low.

@Hearth thanks for pointing that out. Is the following MOSFET suitable for this application: lcsc.com/product-detail/…. I selected it on behalf of your information, selecting low Rdson at the 3,3V logic level.

I updated my question with a MOSFET suggestion based on your inputs. @Bimpelrekkie I updated the resistor and calculated the power dissipation. The resistor should be able to handle 2W. Thanks for pointing that out again!

Regarding the DMN10H120SE MOSFET... If you look at figure 2 on the data sheet, "Typical Transfer Characteristics" you will notice that at 3.3V, the device is barely on, if at all. You really should have minimum 4.5V gate-source. Drain-source resistance for this device is spec'd at 6V and 10V, btw