Can I use + charge 3 LiPo cells in series with 3 separate charge controllers?
Yes, this will work PROVIDED that each cell has it's own PV panel.
See below for cautions. 

Each PV panel has to be connected to Vin and ground of its related charger and NOT to the other PV panels. The batteries can be connected
ground1 = system ground,
V+1-ground2,
V+2-ground3,
V+3=Vout

PV panels should be nominally identical and batteries 9cells) should be nominally identical. 
With this arrangement, charging may occur at slightly different rates due variations in panels, insolation variations (which should be minor), slightly different contamination levels on panels etc. This will lead to one cell being charged before the others and one "coming last". If the differences are as small as they should be this will not matter. If there is more than enough sun the others will catch up in the same day. If panels are at say 90, 85, 75% at the start of a day then as long as discharge is not more than 75% of max possible all will be well. If say cutoff point was 3V/cell thgen monitoring cell voltage individually and stopping discharge when any one cell dropped to 3V would be easy and safe. Rechaarging would then satrt at about 15%, 10% and 0% capacity. On recharge, when the 1st cell reached the CC/CV knee its charge rate would slow (if the charger was able to deliver I_chg_max) and the others would start to catch up. As long as on average there is enough sun to sometimes deliver full charge between discharges the system will self balance, as long as V_cell_moin is not allowed to drop below 3V or whatever lower target voltage is set.

Wherever imbalance may occur - whether n this system or others, discharging without individual cell monitoring risks discharging one or more series cell to below its safe minimum level. Modern electronics makes per-cell monitoring cheap and easy and there is little point in omitting it in systems where imbalance is likely. 

Thusly: