I need to change these three sentences using coordinating or subordinating conjunctions.

The sentences are:
1. Sir Gawain appeared to King Arthur in a dream. He warned of Arthur's impending death.
2. King Arthur told his men to consider the treaty broken if they saw any sword drawn. Sir Modred told his men to do the same.
3. Sir Bedivere threw Excalibur into the lake. A hand appeared out of the lake. The hand caught the sword.

How I changed them:
1. Sir Gawain appeared to King Arthur in a dream and warned him of his impending death.

2. I havent figured this one out yet.

3. Sir Bedivere threw Excalibur into the lake when a hand appeared out of the lake and caught the sword.

Coordinating conjunctions:

http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#top

Subordinating conjunctions:
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions

1 and 3 look good.

I'd make 2 into a compound sentence, using a coordinating conjunction (or a semicolon, but that's not in your directions).

OR ... you could make "King Arthur" and "Sir Mordred" be the subjects of a single clause. How would that turn out, do you suppose?

1 I agree.

2 King Arthur told his men to consider the treaty broken if they saw any sword drawn after/while Sir Modred told his men to do the same.

3 Correct, although you may want to use another subordinating conjunction so you can have 3 different answers. You can use "that".

I could either say King Arthur told his men to consider the treaty broken if they saw any sword drawn and Sir Modred told his men to do the same.

Or King Arthur and Sir Modred both told their men to consider the treaty broken if they saw any sword drawn.

But I am not sure with that second one because I think it modifies the sentence too much

You can use either one for #3 since you have used a coordinating conjunction.

The second one is smoother, less wordy.

Ok thank you so much for your help!

You're welcome!

To change the second sentence using coordinating or subordinating conjunctions, we can use the coordinating conjunction "or":

2. King Arthur told his men to consider the treaty broken if they saw any sword drawn, or Sir Modred told his men to do the same.

To change the third sentence using coordinating or subordinating conjunctions, we can use the subordinating conjunction "when":
3. When Sir Bedivere threw Excalibur into the lake, a hand appeared out of the lake and caught the sword.