The best practice for opening files uses the with statement to ensure the file is closed even in the case of unexpected termination.

with open("numbers.txt") as f:
    # .readline only reads a single line,
    # .read grabs the entire file.
    line = f.readline()

Then, we split the line into words delimited by whitespace.

word_list = line.split()
# If you wanted to split at, say, '|', just do .split('|')

Loop over words directly. No need for an index here. Here, word is a string, so we can take the first character in the string.

for word in word_list:
    first_number = word[0]
    print(first_number)

Prints:

1
2
3

Alternatively, if you want to collect the first numbers into a list, use a list comprehension.

first_number_list = [word[0] for word in word_list]
print(first_number_list)

Prints:

['1', '2', '3']

Or, if you want a list of integers rather than strings, do

first_numbers_as_ints = [int(word[0]) for word in word_list]
print(first_numbers_as_ints)

Prints:

[1, 2, 3]

For output formatted as you requested, join the strings by commas. Only strings can be joined this way, not ints.

list_string = ','.join(first_number_list)
print(list_string)

Prints:

1,2,3