What you’re seeing is called channel hopping, and is mandatory for proper nodes.
If you’re saying it’s only hopping over 3 instead of 8 frequencies, then you’re apparently using ABP. For ABP, for a proper node, you should manually program the 8 frequencies (for EU-868) and the settings for RX2. Most LMiC TTN examples will do that.
To only use a single channel, for testing only, see Can LMIC 1.6 be set up to use a single channel and SF?