Hello,
The device usually chooses cells with the best signal strength, so even if the device connects to B3 + B28 + B40, if the signal quality for the first two bands is not great, it will switch to B40. The carrier aggregation itself cannot be controlled (other than disabling it outright), however, there is one workaround to lock a specific cell as the primary. This way, the primary cell will stay the same, and CA will most likely choose the remaining bands. This has already been discussed here: RUTX50 - Locking 5G-NSA CA bands manually
It should be noted, that this method is not officially supported by us or the modem manufacturer, so if you experience any instability please disable this feature.
Best regards,