diff3 conflict is basically what you always want to see, either by setting it as the default as per stackoverflow.com/questions/27417656/should-diff3-be-default-conflictstyle-on-git:

git config --global merge.conflictstyle diff3

or as a one off:

git checkout --conflict=diff3

With this, conflicts now show up as:

++<<<<<<< HEAD
 +5
++||||||| parent of 7b0f59d (6)
++3
++=======
+ 6
++>>>>>>> 7b0f59d (6)

7b0f59d is the SHA-2 of commit 6.

instead of the inferior default:

++<<<<<<< ours
 +5
++=======
+ 6
++>>>>>>> theirs

We can also observe the current tree state during resolution:

* b4ec057 (HEAD, master) 5
* 0b37c1b 4
| * fbfbfe8 (my-feature) 7
| * 7b0f59d 6
|/
* 661cfab 3
* 6d748a9 2
* c5f8a2c 1

so we understand that we are now at 5 and that we are trying to apply our commit 6

So it is much clearer what is happening:

master changed the code from 3 to 5
our feature changed the code from 3 to 6

and so now we have to decide what the new code is that will put both of these together.

Let's say we decide it is 5 + 6 = 11 and continue rebasing:

git add .
git rebase --continue

We now reach:

++<<<<<<< HEAD
 +11
++||||||| parent of fbfbfe8 (7)
++6
++=======
+ 7
++>>>>>>> fbfbfe8 (7)

and the tree looks like:

* ca7f7ff (HEAD) 6
* b4ec057 (master) 5
* 0b37c1b 4
| * fbfbfe8 (my-feature) 7
| * 7b0f59d 6
|/
* 661cfab 3
* 6d748a9 2
* c5f8a2c 1

So we understand that:

after the previous step we added commit 6 on top of 5
now we are adding 7 on top of the new 6 (which we decided would contain 11)

and after resolving that one we now reach:

* e1aaf20 (HEAD -> my-feature) 7
* ca7f7ff 6
* b4ec057 (master) 5
* 0b37c1b 4
* 661cfab 3
* 6d748a9 2
* c5f8a2c 1

`git mergetool` with `meld` or `kdiff3`

 0  0 

These are good free newbie GUI options:

sudo apt install meld
git mergetool --tool meld

sudo apt install kdiff3
git mergetool --tool kdiff3

https://raw.githubusercontent.com/cirosantilli/media/master/meld.png

https://raw.githubusercontent.com/cirosantilli/media/master/kdiff3.png

Let's make a more interesting conflict:

git-tips-2.sh

#!/usr/bin/env bash

set -eux

add() (
  rm -f f
  for i in `seq 10`; do
    printf "before $i\n\n" >> f
  done
  printf "conflict 1 $1\n\n" >> f
  for i in `seq 10`; do
    printf "middle $i\n\n" >> f
  done
  printf "conflict 2 $2\n\n" >> f
  for i in `seq 10`; do
    printf "after $i\n\n" >> f
  done
  git add f
)

rm -rf git-tips-2
mkdir git-tips-2
cd git-tips-2
git init

for i in 1 2 3; do
  add $i $i
  git commit -m $i
done

add 3 4
git commit -m 4

add 5 4
git commit -m 5

git checkout HEAD~2
git checkout -b my-feature

add 3 6
git commit -m 6

add 7 6
git commit -m 7