J.M.

After a short break I just want to update that I’ve managed to enable the i2c-3 on boot. The truth is that it was too simple… (later I’ll explain what was the mistake).
All I’ve needed to do is:
 
1.  Update the boot device tree (cache/sources/u-boot/v2019.04/arch/arm/dts/sun7i-a20-lamobo-r1.dts) with:
&i2c3 { pinctrl-names = "default"; pinctrl-0 = <&i2c3_pins_a>; status = "okay"; }; I guess that instead of updating the boot device tree, I could also just update /boot/armbianEnv.txt with: overlays=i2c3  
2. Update Lamobo_R1_deconfig file with:
CONFIG_I2C0_ENABLE=y CONFIG_I2C1_ENABLE=y CONFIG_I2C2_ENABLE=y CONFIG_I2C3_ENABLE=y When I just added CONFIG_I2C3_ENABLE=y it didn’t work. I didn’t figure why... The awkward reason I failed to do it in the beginning was because I’m using a script that updates my board on remote (without using SD card). But when I did the burning with SD - it worked !!
The difference between the remote burning script and the burning with SD script is that in the remote script I’m not burning all /dev/mmcblk1, but just /dev/mmcblk1p1 and /dev/mmcblk1p2.
And now I’m only guessing (so you are all welcome to correct my assumption), that the configuration of the i2c on boot is written into mmcblk1boot, and that is why it didn’t work when I used the remote burning script.
 
Thanks to all for the help and support.