Calculate battery consumption in IoT Device, Parameters and Considerations Regarding Bluetooth
Ideally, you would want something to log the power consumption. Bluetooth devices will vary their consumption, depending on if it is idle, if it is transmitting and receiving data and I would guess your ECG sensor would probably fluctuate as well, depending on what it is doing at any given time. Other devices may not be working constantly either.

What you should do is to log the power over a given time while the device is being used at what you would consider under normal conditions and take the average current consumption. You can then use that figure to find out how much it will use in 3 hours or see how long your batteries will last. 

That would be the best way to do it in my opinion, although someone may know better ways!

EDIT**
Just as an example of a BLE device varying its consumption, I found this picture online of someone doing power consumption tests on such a device:


This information was taken from HERE which talks about various modules. As you can see from the picture, depending on what the module is doing, the current peaks at different points so the average will be very important when trying to do these calculations. 

As for accelerometers, one I have used is the LIS3DH and a quick look at the datasheet where it mentions current consumption shows this:


Which again shows that changing states will mean it draws different amounts of current. And as you said you will be getting information every 5 seconds, I doubt it will be staying in 1 state the entire time!