Arduino sheild compatability

​I am planning on making a project where a call is placed depending on the accelerometer values. 
I need to send a preconfigured message and gps data to a predefined number. 
So i would like to know whether I can use the gsm shield, Accelerometer sensor, and gps sensor, together in arduino uno r3. Will the sheilds be using the same pins? 
How can I make sure before buying the shields?
If the shields pins will overlap, whats the other way out?

You don,t necessarily need shields for each of those functions. Accelerometer can be digital (I2C) or analog. Analog accel will go to AO - A6, a pin for each axis. The I2C accels use pins A4 and A5. My GPS (Neo 6M) uses software serial so any 2 digital pins for that. The GSM shields that I have seen use software serial on pins 2 and 3 by default, but, on some boards, may be changed. So I think that you will not have trouble in terms of pin usage. Can't speak about library compatibility, though.

thanks for the fast reply.

I would like to know whether i can use these items together

this accelerometer from amazon

along with this gps shield

and finally this gsm module

If not, what is your recommendation in going about this project?

Strictly speaking only the GSM unit is a shield. The GPS and accel are modules. The GSM shield seems to use pins 0 and 1 for communication. The Uno uses those pins for uploading your program so you will need to unplug the shield to program your Uno and then plug the shield back in to use it. I would look for a shield that uses other pins for (software) serial comms. The accel will use 3 analog pins and the GPS will use 2 pins set up for software serial. I think that if you find a GSM sheild that doesn't use the hardware serial pins you can use those together.

how can I identify a gsm shield that doesn’t use the hardware serial pins? and how did you identify that the previous gsm shield uses pins 0 and 1 for communication ? is it from the picture?

what about these two gsm shields

Whats the difference between SIM300 and SIM 900?

Thanks a lot for your prompt and valuable response!

To find which pins the GSM shield used I found the board on the manufacturers web site. As to the difference between 300 and 900, I have no clue. I have no experience or knowledge of GSM.

The linked are all modules and so can be connected via software serial to pins of your choosing. The problem with the shield is that it plugs into the hardware serial port and you have no choice as to the pins to control the shield. With the modules you have the choice.

I should mention that if you have 2 software serial ports (GPS and GSM) only one can receive or transmit at a time, so your code needs to allow for that.

I am planning to purchase this,SIM900A I hope I will be able to connect it along with the gps and accelerometer sensors.