If I finish davening Shemonah Esrei and I'm about to take three steps back and do the bowing procedure, can I first do something like pick up my cell phone and put it in my pocket or close a sefer or anything else that doesn't involve talking or moving before I take the steps backwards?
According to Halachipedia Taking Three Steps Back the answer appears to be No. The three steps back is part of the shmoneh esrai and should not be interrupted.
After Shemonei Esrei, one should make take three steps backwards while bowing like a servant parting from his master. If one didn't do so, it is as if one didn't pray.
Gemara Yoma 53b, Tur 123:1, Kaf HaChaim 123:1
Note that the actual quote from the gemara is
The Gemara comments that this was also taught in a baraita: One who prays must take three steps backward upon concluding his prayer and then recite: Peace. And if he did not do so, it is better for him not to have prayed, as his actions are disrespectful toward God.
Resuming the citation from Halachipedia
There’s an absolute obligation to take 3 steps back after Shmoneh Esrei and wait there until the Sheliach Tzibbur gets to Kedusha or at least starts Chazarat HaShas. 
An individual who finishes Shmoneh Esrei should take take three steps back anf wait there the time it takes for the שליח Tzibbur to reach Kedusha or in extenuating circumstances at least the time it takes to walk 4 amot (3 seconds or so).
If one doesn’t wait after taking the 3 steps back it appears that one didn’t take the 3 steps back really in order to depart from Hashem respectfully. 
It’s proper to take three steps forward only after waiting the proper time (subsequent to taking three steps back).