/** ----- Answer ----- **/[1. Before playing the tennis for the third time]
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The correct way of saying would be..
テニスを【する/やる】のはこれが３回目なんだ。
or テニスは２回しか【した/やった】ことがないんだ。
If you want to emphasize "only 3 times", you can add まだ in front of これが and the sentence would be..
テニスを【する/やる】のはまだこれが３回目なんだ。
or テニスはまだ２回しか【した/やった】ことがないんだ。
[2. After playing the tennis for the third time]
■note■
I used symbol 【blah/blahh】 for the words you cannot miss out but you can choose one out from the words listed without altering the meaning,
symbol （blah） for the words you may add if you want to (to emphasize, to make the meaning clear, etc.).
/** ----- Follow-ups ----- **/■ する and やる (past tense : した and やった) have almost the same meaning so you can use which one you feel like using.
ie.
ぜひ一緒に【やりましょう/しましょう】。（Let's do together.）
サッカー【しません/やりません】か？（Want to play football?）
山登り【行きません/しません】か？（Let's go to a mountain hike.）
一緒に食事【に行きません/しません】か？（Let's go to eat together.）
※Like in the last 2 sentences, there are some cases you can't use やる.
By just remembering that nuance of する is more passive, and やる is more active,
you can figure out that walking and eating is not so much active stuff enough to use やる.
■ In your sentence you have used だけ trying to emphasize the word "only", but まだ should be used here.
だけ should be used when you want to restrict (３回だけ＝only three times; ３回目だけ＝only the third time).
ie.
君に残されたチャンスはあと３回だけだよ。（You have only 3 chances left.）
１回だけでいいから、お願い！（Just once is enough! Pretty pleeease..!）
ではゲームを始めようか。各自２回テストプレイをしてから公式記録をとる。記録として残るのは３回目のプレイだけだよ。
（Let's start the game. You have 2 tries playing the game and then we will move on to play for the official record. Only the third play would be recorded officially.）
まだ means "only .. until now" and it just shows a state of something, and there is no nuance of restriction.
ie.
まだ電車が来ていない。（Train hasn't come yet.）
日本に来てまだ１年目です。（This is only my first year since I came to Japan.）
まだ１回しかトライしてないからあと２回チャンスがあるぞ。（I've only tried once so I have still 2 more tries/chances left.）
