Or can I, say start with spellforce 3, finish that and then come back to 1 and do that and then do 2? Or any combination of them and their expansions?

Since the 3 is a prequel, in teory the order should be this:

Spellforce 3: Reforced (or "Legacy" because are the same game, but "Reforced" uses a newer engine)
Spellforce 3: Soul Harvest
Spellforce 3: Fallen God

Spellforce: The Order of Dawn
Spellforce: Breath of Winter
Spellforce: Shadow of the phoenix
(This last story works as an aggregator ending for both the 2 previous campaigns, and could be played twice in a row with both the previous characters)

Spellforce 2: Shadow Wars
Spellforce 2: Dragon Storm
Spellforce 2: Faith in Destiny
Spellforce 2: Demons of The Past

If you play SF1 and SF2 before the SF3 you will recognize some (rare) references and jokes from the previous games anyway...

SF2 has a lot of references to SF1.

Be aware that the gameplay changes A LOT from SF3 to the previous 2 series, and even those are pretty different from each others.


EDIT:
added an explanation in the list about the two versions of SP3

Ugh, another prequel - I am SO fed up with that! A damn shame I just learned that, because once they managed to figure out how to make the SP3 releases make sense, I was planning to give it a go. I still don't know what SP3: "Reforced" (...?) is.

What a damn shame that is too - I own like 7 copies of SpellForce 1, retail and digital, and I have spent hours and hours and HOURS truly enjoying that game, with the expansions. I can pretty much play through SP1 in my sleep.

...And then came SpellForce 2, with a completely different gameplay, game breaking bugs...and no runestones, the one concept I had never seen before in a game of this genre. The one thing, that to me at least, made SpellForce 1 unique.

Whatever they did with SP2, they completely ruined it as far as I am concerned. Like, literally ruin everything with it (except the soundtrack, which was still good).

I made it through the main game, and I gave up after that. The game held zero interest for me, with no memorable moments (also, the whole SP2 expansion release was such a mess - and same with SP3 - I don't even know what the main game is called).

"Spellforce 3: Reforced" is "Spellforce 3" with a newer engine (the one that was improved in the next games of the serie).

Because the new engine also had some changes in the gameplay, the original "Spellforce 3" was made still available with the name "Spellforce 3: Legacy".

So these are both the first game of the SP3 serie:
"Spellforce 3: Reforced" (new engine)
"Spellforce 3: Legacy" (original engine)

For the rest I feel almost the same as you, SP2 had some improvements in gameplay, but it was over simplified and lost a lot of the deepness of the first serie... I also didn't like the fact that is actually possible to obtain a "game over"... in the first game was impossible to die, so it was more relaxing...
Anyway SP2 has a third person view and uses the same "Click & Fight" interface to give order to your troops, so I still liked it.

SP3 instead uses a different way to give attack/spell orders that I didn't like very much... I found very difficult to select targets inside crowd of people fighting... I never had this issue in the previous games.
There's a thing anyway where SP3 is way better than the previous series, it is the story, really deep and detailed, long with a lot of twists, and also the interactions with your followers, this last thing is improved even more in "Soul Harvest" and "Fallen God".
"Fallen God" in particular is like 90% story/dialogs and 10% gameplay, I liked it, it is well made and was a cool journey, but I can't see a lot of replayability.