I bought a fast charger Ldino has QC3 18W for tablet Asus s3 10 but the charging is still slow !! I only had to use old usb cable because this tablet has usb c and the charger cable come with micro usb so is this the reason ?
The protocols used on both devices have to be the same and compatible. Charger and device have to be able to take the extra power. Poor cable can heat up and reduce charging rate.
The way it fast charges is to force higher power delivery over communicating with the device to agree that it can take the up to 20V and 18W. They need the higher voltage to transfer higher power and lower losses at the cable, most USB cables (and even other) are pretty trash no matter what you buy.
I think the problem is your device is not QC3, it only says so on gsmarena but as far as I know QC3 is not on MediaTek? You need Qualcomm's Snapdragon for QC3. Sadly each company has tried to proprietarize their corner of the market and I don't think USB-PD is widely used yet that would work on any standardized device.
As far as I can find you can buy a fast charging Asus charger for this device but it's not included with the device, it's bought separately.
If you ask me, you could probably try Mediatek compatible fast charger, it's likely just Asus being fake (misleading customers with a more popular buzzword/brandword) and writing quick charge 3.0 when they should have written pump express 3.0.
Asus advertise this in all pages like their product page and even on Amazon. I tried some apps that measure the amp but it reported that my tablet does not has power profile so I can not get the reading from a software. btw I used the origingal cable and I dont have Asus QC3 here only that ldino.
Yeah their marketing probably wrote what they want not what it actually supports. Dig around and you will find it what charger works and what fast charging is supported by the hardware used in the device.
There are USB meters that can show you the voltage and current being transferred (if you don't have a multimeter for measuring, those meters are easy to use if one plans to test and test a lot with their different devices). Don't bother with software apps.
Definitely try a quality USB cable (you mentioned it was old, but was it a brand name, or a cheap one?). Cables do make a difference, in both charging speed, and data transfer speed. Always make sure to use a quality one if possible.