"We have heard of Moab's pride - her overweening pride and conceit, her pride and insolence - but her boasts are empty." NIV translation
The pride of the Moabites would not allow them to accept Israel's help or God's mercy. This type of pride is the root of most sin and is the reason that Satan was kicked out of heaven. A prideful heart says: I don't need God; I can do it myself.
"Therefore the Moabites wail, they wail together for Moab. Lament and grieve for the men of Kir Hareseth.
The fields of Heshbon wither, the vines of Sibmah also. The rulers of the nations have trampled down the choicest vines, which once reached Jazer and spread toward the desert. Their shoots spread out and went as far as the sea. So I weep, as Jazer weeps, for the vines of Sibmah. O Heshbon, O Elealeh, I drench you with tears!
The shouts of joy over your ripened fruit and over your harvests have been stilled. Joy and gladness are taken away from the orchards; no one sings or shouts in the vineyards; no one treads out wine at the presses, for I have put an end to the shouting." NIV translation
The result of rejection of God's mercy is the removal of His hand of provision. As we see in this passage, the Moabites refused God's offer of mercy and so He removed the provision of their vineyards. Moab was known for its wine but did not acknowledge that it was the hand of God that caused the vines to grow and to produce a crop. When He removed His hand of provision, the harvest ended and they mourned over the fields but still did not see their sin of pride.
"My heart laments for Moab like a harp, my inmost being for Kir Hareseth. When Moab appears at her high place, she only wears herself out; when she goes to her shrine to pray, it is to no avail." NIV translation
While God was offering mercy the Moabites were still chasing idols and so it grieved God. The same thing happens today when we, as Christians, put other things before our worship of God.
"This is the word the Lord has already spoken concerning Moab. But now the Lord says: 'Within three years, as a servant bound by contract would count them, Moab's splendor and all her many people will be despised, and her survivors will be very few and feeble.'" NIV translation
God had already known that the Moabites were going to reject His mercy and said what was going to happen because of it. Now, He gives them the timeframe for the fulfillment of the judgment upon them. A "servant bound by contract" counts every day and knows the exact day when their time of bondage will be up. Likewise, God said that it would be precisely three years until the Moabites were judged.
We do not know when Jesus is coming back for the final judgment but, as it was with the Moabites, we can choose to accept His mercy and grace or we, too, will face sure judgment.