user28330: So I've got three boards that I'm using to test this (modified) code: 
int sensor1Pin = A0;
int sensor2Pin = A1;
int sensor3Pin = A2;


float sensors[] = {0,0,0};

void setup(){
Serial.begin(9600);
}

void loop(){

sensors[0] = analogRead(sensor1Pin);
sensors[1] = analogRead(sensor2Pin);
sensors[2] = analogRead(sensor3Pin);

Serial.print(sensors[0]);
Serial.print(",");
Serial.print(sensors[1]);
Serial.print(",");
Serial.println(sensors[1]);
}

An Uno, a Feather M0, and an MKR1000. 
The code will only work on the Uno because it's AVR-based whereas the other two boards are SAMD. When running on a feather, you get this error:
/home/ghylama/Arduino/libraries/SoftwareSerial_master/SoftwareSerial.cpp:260:2: error: #error This version of SoftwareSerial supports only 20, 16 and 8MHz processors
     #error This version of SoftwareSerial supports only 20, 16 and 8MHz processors

However, the resolution of the Uno is too low for my purposes (its DAC has a resolution of 10, whereas I need a resolution of 12, minimum). Are there any libraries/workarounds that I can use for this purpose?
I've tried SoftwareSerial, NeoSWSerial, and this one.

user30175: I will restate the problem as I have understood it: You want more Serial objects, aka more UART ports.
On your Adafruit Feather M0 with its SAMD21G18 Cortex-M0+ MCU, you have six SERCOM peripherals which you can arbitrarily configure to be a UART, I2C or SPI port. Each peripheral can have its connections routed to several different pins on the board. It is a very versatile piece of hardware. 
Don't use a SoftwareSerial library made for AVR MCUs when you have the hardware. Adafruit's guide shows how you can reconfigure those SERCOMs to get additional Serial objects. 
https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-ports?view=all#creating-a-new-serial

28330: <p>So I've got three boards that I'm using to test this (<a href="http://www.hackerscapes.com/2014/11/how-to-save-data-from-arduino-to-a-csv-file-using-processing/" rel="nofollow noreferrer">modified</a>) code: </p>

<pre><code>int sensor1Pin = A0;
int sensor2Pin = A1;
int sensor3Pin = A2;


float sensors[] = {0,0,0};

void setup(){
Serial.begin(9600);
}

void loop(){

sensors[0] = analogRead(sensor1Pin);
sensors[1] = analogRead(sensor2Pin);
sensors[2] = analogRead(sensor3Pin);

Serial.print(sensors[0]);
Serial.print(",");
Serial.print(sensors[1]);
Serial.print(",");
Serial.println(sensors[1]);
}
</code></pre>

<p>An Uno, a Feather M0, and an MKR1000. 
The code will only work on the Uno because it's AVR-based whereas the other two boards are SAMD. When running on a feather, you get this error:</p>

<pre><code>/home/ghylama/Arduino/libraries/SoftwareSerial_master/SoftwareSerial.cpp:260:2: error: #error This version of SoftwareSerial supports only 20, 16 and 8MHz processors
     #error This version of SoftwareSerial supports only 20, 16 and 8MHz processors
</code></pre>

<p>However, the resolution of the Uno is too low for my purposes (its DAC has a resolution of 10, whereas I need a resolution of 12, minimum). Are there any libraries/workarounds that I can use for this purpose?</p>

<p>I've tried SoftwareSerial, NeoSWSerial, and <a href="https://github.com/arduino-org/Arduino/commit/1364019868e2ad044c87dd0f143052a92eebda98" rel="nofollow noreferrer">this one</a>.</p>
 30175: <p>I will restate the problem as I have understood it: You want more Serial objects, aka more UART ports.</p>

<p>On your Adafruit Feather M0 with its SAMD21G18 Cortex-M0+ MCU, you have six SERCOM peripherals which you can arbitrarily configure to be a UART, I2C or SPI port. Each peripheral can have its connections routed to several different pins on the board. It is a very versatile piece of hardware. </p>

<p>Don't use a SoftwareSerial library made for AVR MCUs when you have the hardware. Adafruit's guide shows how you can reconfigure those SERCOMs to get additional <code>Serial</code> objects. </p>

<p><a href="https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-ports?view=all#creating-a-new-serial" rel="nofollow noreferrer">https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-ports?view=all#creating-a-new-serial</a></p>
