It turns out that there are some missing information -or I didn't find it- regarding to the SPI module.
The information available in the TF-Card page doesn't apply to the AtomS3 and there is no TF-CARD module in the Unit selection insdide UIFlow.
I compiled and ran the example for the Arduino IDE and then tried again to build a working example using microPython.
To whom it may interest, this is the code that works with my device:
import os
from machine import SDCard
from machine import Pin
sd = SDCard(slot=3, miso=Pin(8), mosi=Pin(6), sck=Pin(7), cs=Pin(5))
sd.info()
os.mount(sd, '/sd')
print("SD card mounted at \"/sd\"")
print(os.listdir('/sd'))