Daniel's Visual Phasing Tool  beta v 1.1 (Ancestry, 23andMe, MyHeritage)
Unzipped raw DNA text file for person A:
Unzipped raw DNA text file for person B:
Unzipped raw DNA text file for person C:

Animation: How to use Visual Phaser
To get started select the raw DNA .txt or .csv files of 3 people you'd like to compare above, then click the button.
What is this?
This is a tool to help with the task of performing DNA visual phasing between 3 people.

To use this tool, you must have access to the raw DNA data files of the 3 people you would like to compare. Usually this type of comparison is done using 3 people who are all full siblings to each other, though it doesn't have to be.
How does it work?
This tool takes the raw DNA data of 3 individuals and compares them to each other, 1 chromosome at a time. The 3 people are referred to as A, B, & C. The tool generates 3 horizontal representations of chromosome comparisons between persons A/B, A/C, and B/C. At each location along the chromosome, if the two people are a full match (i.e. they match on both alleles), a green vertical line is drawn. If they're only a half match (i.e. they match on one of the 2 alleles), a yellow vertical line is drawn. If they don't match at all (i.e. they have neither allele in common), a red vertical line is drawn.

At first, this type of comparison may not seem very helpful. But when the lines are looked at as blocks of color instead of as individual components, the differences between the 3 comparison bars are more easily revealed. These differences are important in genetic genealogy because they indicate positions along the chromosome where the pattern of inheritance between the 2 people has changed; effectively grouping portions of the chromosome together into segments. In the case of full siblings, this visualization can aid in determining from which of the 4 grandparents a segment of DNA was inherited.

After selecting your DNA files, the graphic representation of their comparisons will be shown. First move the "Segment Expansion" slider to get the best view of your chromosome's colored segments. Then click "Add A New Alignment Bar", and drag it to a position where a color change occurs in at least 2 of the chromosome comparisons. Above each Alignment Bar, you will see the SNP position number in Megabases (i.e. 25.6 = position 25,600,000 along the given chromosome).
Is it secure?
Yes! This tool does all of the processing and displaying of the DNA data natively in your web browser. That means no data or files are uploaded or transferred anywhere. It all happens right on your own computer.

Special thanks to David Pike and his awesome DNA utilities without which I wouldn't have been able to make this tool. I don't know him, but if you do, please buy him a beer on my behalf.