Convert your text files to BED files, sort them with BEDOPS sort-bed and run BEDOPS bedops --intersect on them to get the intervals common to them.

For example, file1.txt:

68476204: 9-50, 55-75, 80-132
NC_23987: 2-22, 1001-1085
68473073: 1-8
68485121: 1-10, 20-55

becomes file1.bed (when sorted):

68473073   1    8
68476204   9    50
68476204   55   75
68476204   80   132
68485121   1    10
68485121   20   55
NC_23987   2    22
NC_23987   1001 1085

And so on.

To convert from text to BED, you could use a Python script:

#!/usr/bin/env python                                                                                                                                                                   

import sys

for line in sys.stdin:
    (chr, intervals_str) = line.rstrip('\n').split(':')
    for interval in intervals_str.replace(' ', '').split(','):
        (start, stop) = interval.split('-')
        sys.stdout.write('%s\t%s\t%s\n' % (chr, start, stop))

Then:

$ convert.py < file1.txt | sort-bed - > file1.bed

Etc.

Once you have BED files, you can do set operations on the sorted BED files:

$ bedops --intersect file1.bed file2.bed file3.bed > answer.bed

Once you have the intersection of intervals, you can filter that result with awk based on interval length:

$ awk '($3-$2 >= 15)' answer.bed > filtered_answer.bed

Etc.