First of all, it is not viable testing the new progression system in any of the official testlive server such as Combat_TestLive_EU3_PVE because the XP rate multiplier and the Harvesting rate multiplier are set to 5x. So, I created a new game/new character in singleplayer and set everything in the setting as default.
Low level low tier
Here are some of the XP bar screenshots
XP bar before harvesting
XP bar after harvesting stone 12x hits
XP bar before a combat
XP bar after killing an imp with basic attack
XP bar before a journey step
XP bar after using a torch journey step
Conclusion: in my opinion, in a low level and low tier, harvesting is still considered as the lowest XP gainer while the other 2 i.e. combat and journey step competing to be the highest XP gainer of the game. Harvesting progressing change is not yet significant enough to be used as a main XP gainer for players. The journey step is still considered as a high XP gainer and with a boost on the combat/killing NPC, both of them are most probably equally high. The combat progressing change, though, can be considered significant enough as a main XP gainer for players, because it is slightly higher than journey step.